Allow customization of request headers in HttpHandler

[sarahsnow1]

Currently there isn't a way to add custom request headers in HttpHandler subclasses because the underlying http client is private. The Android SDK will need this ability when implementing mapzen/android#382 and EM relies on the ability to customize headers when adding the "do not track" header https://github.com/mapzen/eraser-map/blob/master/app/src/main/kotlin/com/mapzen/erasermap/model/TileHttpHandler.kt#L19

I propose adding a header Map parameter to onRequest and updating the implementation to construct a Request using this map:

public boolean onRequest(String url, Map<String, String> headers, Callback cb) {
  Request request = new Request.Builder()
    .url(url)
    .headers(Headers.of(headers))
    .build();
  okClient.newCall(request).enqueue(cb);
  return true;
}

[matteblair]

We could also just make the HTTP client member protected to allow access for subclasses. This would enable header customization as well as any future tweaks that we might want for URL requests. Do you see major downsides to that approach @sarahlensing?

[sarahsnow1]

@matteblair I can't think of any, I think that is a fine way forward.

[msmollin]

FWIW on iOS I just exposed the underlying NSURLSession HTTP Headers object (mostly because I hadn't seen this thread) - #1578

It would be easier to also expose the NSURLSession session object to the Obj-c subclasses so this could be modified as well. If that's the path we're set on, I can make that change as well.

[tallytalwar]

Closing this as #1587 takes care of providing some flexibility to extend HttpHandler to customize headers.