Add Memento HTTP actor

This adds a Memento actor to the HTTP bus that has a priority over the other HTTP actors and will intercept requests to perform datetime negotiation when applicable.

The datetime can be set via the `-d` command line option.

Closes #79

packages/actor-http-memento/README.md

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+# Comunica Memento Http Actor
+
+A comunica Memento Http Actor.

packages/actor-http-memento/components/Actor/Http/Memento.jsonld

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+{
+  "@context": [
+    "https://linkedsoftwaredependencies.org/bundles/npm/@comunica/actor-http-memento/^1.0.0/components/context.jsonld",
+    "https://linkedsoftwaredependencies.org/bundles/npm/@comunica/bus-http/^1.0.0/components/context.jsonld"
+  ],
+  "@id": "npmd:@comunica/actor-http-memento",
+  "components": [
+    {
+      "@id": "cahm:Actor/Http/Memento",
+      "@type": "Class",
+      "extends": "cbh:Actor/Http",
+      "requireElement": "ActorHttpMemento",
+      "comment": "A comunica Memento Http Actor.",
+      "parameters": [
+        {
+          "@id": "cahm:Actor/Http/Memento/mediatorHttp",
+          "comment": "The HTTP mediator",
+          "required": true,
+          "unique": true
+        }
+      ],
+      "constructorArguments": [
+        {
+          "@id": "cahm:Actor/Http/Memento/constructorArgumentsObject",
+          "extends": "cbh:Actor/Http/constructorArgumentsObject",
+          "fields": [
+            {
+              "keyRaw": "mediatorHttp",
+              "value": "cahm:Actor/Http/Memento/mediatorHttp"
+            }
+          ]
+        }
+      ]
+    }
+  ]
+}

packages/actor-http-memento/components/components.jsonld

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+{
+  "@context": "https://linkedsoftwaredependencies.org/bundles/npm/@comunica/actor-http-memento/^1.0.0/components/context.jsonld",
+  "@id": "npmd:@comunica/actor-http-memento",
+  "@type": "Module",
+  "requireName": "@comunica/actor-http-memento",
+  "import": [
+    "files-cahm:components/Actor/Http/Memento.jsonld"
+  ]
+}

packages/actor-http-memento/components/context.jsonld

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+{
+  "@context": [
+    "https://linkedsoftwaredependencies.org/bundles/npm/componentsjs/^3.0.0/components/context.jsonld",
+    {
+      "npmd": "https://linkedsoftwaredependencies.org/bundles/npm/",
+      "cahm": "npmd:@comunica/actor-http-memento/",
+      "files-cahm": "cahm:^1.0.0/",
+
+      "ActorHttpMemento": "cahm:Actor/Http/Memento"
+    }
+  ]
+}

packages/actor-http-memento/index.ts

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+export * from './lib/ActorHttpMemento';

packages/actor-http-memento/lib/ActorHttpMemento.ts

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+import { ActorHttp, IActionHttp, IActorHttpOutput } from "@comunica/bus-http";
+import { ActionContext, IActorArgs, IActorTest, Mediator } from "@comunica/core";
+import "isomorphic-fetch";
+import * as parseLink from 'parse-link-header';
+
+/**
+ * A comunica Memento Http Actor.
+ */
+export class ActorHttpMemento extends ActorHttp {
+
+  public readonly mediatorHttp: Mediator<ActorHttp,
+    IActionHttp, IActorTest, IActorHttpOutput>;
+
+  constructor(args: IActorHttpMementoArgs) {
+    super(args);
+  }
+
+  public async test(action: IActionHttp): Promise<IActorTest> {
+    if (!(action.context && action.context.has(KEY_CONTEXT_DATETIME) &&
+          action.context.get(KEY_CONTEXT_DATETIME) instanceof Date)) {
+      throw new Error('This actor only handles request with a set valid datetime.');
+    }
+    if (action.init && new Headers(action.init.headers || {}).has('accept-datetime')) {
+      throw new Error('The request already has a set datetime.');
+    }
+    return true;
+  }
+
+  public async run(action: IActionHttp): Promise<IActorHttpOutput> {
+    // Duplicate the ActionHttp to append a datetime header to the request.
+    const init: RequestInit = action.init ? {...action.init} : {};
+    const headers: Headers = init.headers = new Headers(init.headers || {});
+
+    if (action.context && action.context.has(KEY_CONTEXT_DATETIME)) {
+      headers.append('accept-datetime', action.context.get(KEY_CONTEXT_DATETIME).toUTCString());
+    }
+
+    const httpAction: IActionHttp = { context: action.context, input: action.input, init };
+
+    // Execute the request and follow the timegate in the response (if any).
+    const result: IActorHttpOutput = await this.mediatorHttp.mediate(httpAction);
+
+    // Did we ask for a time-negotiated response, but haven't received one?
+    if (headers.has('accept-datetime') && result.headers && !result.headers.has('memento-datetime')) {
+      // The links might have a timegate that can help us
+      const links = result.headers.has('link') && parseLink(result.headers.get('link'));
+      result.body.cancel();
+      if (links && links.timegate) {
+        // Respond with a time-negotiated response from the timegate instead
+        const followLink: IActionHttp = { context: action.context, input: links.timegate.url, init };
+        return this.mediatorHttp.mediate(followLink);
+      }
+    }
+
+    return result;
+  }
+}
+
+export interface IActorHttpMementoArgs
+  extends IActorArgs<IActionHttp, IActorTest, IActorHttpOutput> {
+  mediatorHttp: Mediator<ActorHttp,
+  IActionHttp, IActorTest, IActorHttpOutput>;
+}
+
+/**
+ * @type {string} Context entry for the desired datetime.
+ */
+export const KEY_CONTEXT_DATETIME = '@comunica/actor-http-memento:datetime';

packages/actor-http-memento/package.json

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+{
+  "name": "@comunica/actor-http-memento",
+  "version": "1.0.0",
+  "description": "A memento http actor",
+  "lsd:module": "https://linkedsoftwaredependencies.org/bundles/npm/@comunica/actor-http-memento",
+  "lsd:components": "components/components.jsonld",
+  "lsd:contexts": {
+    "https://linkedsoftwaredependencies.org/bundles/npm/@comunica/actor-http-memento/^1.0.0/components/context.jsonld": "components/context.jsonld"
+  },
+  "lsd:importPaths": {
+    "https://linkedsoftwaredependencies.org/bundles/npm/@comunica/actor-http-memento/^1.0.0/components/": "components/"
+  },
+  "main": "index.js",
+  "typings": "index",
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/comunica/actor-http-memento.git"
+  },
+  "keywords": [
+    "comunica",
+    "actor",
+    "http",
+    "memento"
+  ],
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/comunica/actor-http-memento/issues"
+  },
+  "homepage": "https://github.com/comunica/actor-http-memento#readme",
+  "files": [
+    "components",
+    "lib/**/*.d.ts",
+    "lib/**/*.js",
+    "index.d.ts",
+    "index.js"
+  ],
+  "dependencies": {
+    "parse-link-header": "^1.0.1",
+    "isomorphic-fetch": "^2.2.1"
+  },
+  "peerDependencies": {
+    "@comunica/bus-http": "^1.0.0",
+    "@comunica/core": "^1.0.0"
+  },
+  "devDependencies": {
+    "@comunica/core": "^1.1.0",
+    "@comunica/bus-http": "^1.1.0"
+  },
+  "jest": {
+    "transform": {
+      "^.+\\.ts$": "<rootDir>/../../node_modules/ts-jest/preprocessor.js"
+    },
+    "transformIgnorePatterns": [
+      "[/\\\\]node_modules[/\\\\].+\\.(js|jsx)$"
+    ],
+    "testRegex": "(/test/.*|(\\.|/)(test|spec))\\.ts$",
+    "moduleFileExtensions": [
+      "ts",
+      "js"
+    ],
+    "collectCoverage": true
+  },
+  "scripts": {
+    "test": "node \"../../node_modules/jest/bin/jest.js\" ${1}",
+    "test-watch": "node \"../../node_modules/jest/bin/jest.js\" ${1} --watch",
+    "lint": "node \"../../node_modules/tslint/bin/tslint\" lib/**/*.ts test/**/*.ts --exclude '**/*.d.ts'",
+    "build": "node \"../../node_modules/typescript/bin/tsc\"",
+    "validate": "npm ls"
+  }
+}