Omit base path mapping from the path in a proxied request

[piotrkubisa]

First, I'd like to say thank you @tj for your work!

I have been using apex/gateway for last few days while I was taking care of moving serverfull to serverless API. I have noticed two things, for which I've addressed fixes in this PR submission:

1. Removing addr string which was passed as a parameter to ListenAndServe but I haven't noticed any usage of this variable.
2. Making possible to work with both: custom Domain Name with a Base Path Mapping and default, auto-generated execute-api API endpoints.

Explanation of problem (2): Payload which you can receive via execute-api and this on your custom domain name with Base Path Mapping set is not consistent. Let /v2 will be our BASE_PATH, and also let / (ListItems) and /{id} (GetItem) routes be registered in API Gateway and http.Handler. If you will call execute-api with GET https://{apiId}.execute-api.{aws}.com/{stage}/v2/ you will see response for ListItems, but if you use api.example.com as domain name and if you call it by GET https://api.example.com/v2/ you will be calling GetItem where v2 will be mapped parameter for /{id}.

TL;DR:

• GET https://{apiId}.execute-api.{aws}.com/{stage}/v2/ - call correctly ListItems handler
• GET https://api.example.com/v2/ - call incorrectly GetItem handler

Similar issue was reported in:

• req.path is different between default API Gateway domain and custom domain CodeGenieApp/serverless-express#86 and Add options to aws-serverless-express module to strip base path CodeGenieApp/serverless-express#98
• custom domain name with base path aws/serverless-java-container#34

And also implemented in official APIG-Lambda proxy - awslabs/aws-lambda-go-api-proxy: https://github.com/awslabs/aws-lambda-go-api-proxy/blob/5aaec6761420bbbd4b1f4cf9e2d6eff552413986/core/request.go#L74-L96

Sample usage (example from actual Readme.md will be valid too):

package main

import (
	"fmt"
	"log"
	"net/http"
	"os"

	"github.com/apex/gateway"
	"github.com/go-chi/chi"
)

func main() {
	// Feel free to replace chi.Router with any http.Handler
	r := chi.NewRouter()
	r.Get("/", hello)

	// i.e. BASE_PATH was provided as env variable in SAM CFN definition
	// base path could be a `v1`, `users` or just an empty string
	g := &gateway.Gateway{
		BasePath: os.Getenv("BASE_PATH"),
		Handler:  r,
	}
	log.Fatal(g.ListenAndServe())
}

func hello(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintln(w, "Hello World from Go")
}

To sum up:

• Signature of gateway.ListenAndServe does not change.
• If somebody is interested in BasePathMapping then he just need to define it via struct field, which is very straightforward in Go.
• Defaults will be okay - non breaking anything.
• addr string is marked as ignored via blank identifier - again, non breaking signature Unfortunately, in the example presented in Readme.md tells to use port-binding mapping, which may lead to misinformation, what exactly this variable do (err.. nothing).
• Allows creating simple unit tests (if somebody is not interested in SAM local), thanks to separated anonymous function as a Gateway.Serve method.

Note: Registering route twice for different resources is workaround, which works on both type of endpoints.

[piotrkubisa]

I am closing this PR, because I think it is better to configure the AWS Lambda function to work only with one type of the endpoint (custom domain or *.execute-api) and define a sub-routing with base path or not. Generally speaking: Less conditions = less problems.