feat: client headers for CubejsApi (#242). Thanks to @ferrants!

* client headers for CubejsApi

* headers docs

* default for headers

Fixes #241

docs/Cube.js-Frontend/@cubejs-client-core.md

@@ -32,6 +32,8 @@ Can be an async function without arguments that returns API token.
 - `options` - options object.
 - `options.apiUrl` - URL of your Cube.js Backend.
 By default, in the development environment it is `http://localhost:4000/cubejs-api/v1`.
+- `options.headers` - optional object of additional headers to apply to requests
+
 
 **Returns:** [CubejsApi](#cubejs-api)
 

packages/cubejs-client-core/src/HttpTransport.js

@@ -2,9 +2,10 @@ import fetch from 'cross-fetch';
 import 'url-search-params-polyfill';
 
 class HttpTransport {
-  constructor({ authorization, apiUrl }) {
+  constructor({ authorization, apiUrl, headers = {} }) {
     this.authorization = authorization;
     this.apiUrl = apiUrl;
+    this.headers = headers
   }
 
   request(method, params) {
@@ -16,7 +17,7 @@ class HttpTransport {
 
     const runRequest = () => fetch(
       `${this.apiUrl}/${method}${searchParams.toString().length ? `?${searchParams}` : ''}`, {
-        headers: { Authorization: this.authorization, 'Content-Type': 'application/json' }
+        headers: Object.assign({ Authorization: this.authorization, 'Content-Type': 'application/json' }, this.headers)
       }
     );
 

packages/cubejs-client-core/src/index.js

@@ -35,9 +35,11 @@ class CubejsApi {
     options = options || {};
     this.apiToken = apiToken;
     this.apiUrl = options.apiUrl || API_URL;
+    this.headers = options.headers || {};
     this.transport = options.transport || new HttpTransport({
       authorization: typeof apiToken === 'function' ? undefined : apiToken,
-      apiUrl: this.apiUrl
+      apiUrl: this.apiUrl,
+      headers: this.headers
     });
     this.pollInterval = options.pollInterval || 5;
   }