Support passing FormData as data to http requests.

Fixes #1116
Fixes #1125

When passing a FormData object to Dart's built-in [HttpRequest.request](https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart-dom-html.HttpRequest#id_request) method, it will set content-type accordingly: `Content-Type:multipart/form-data; boundary=----WebKitFormBoundaryfuRomRQ2mNCfdxYb`

This PR does two things: 

- Prevents the HttpService from attempting to serialize FormData as JSON
- Do not apply default Content-Type header if the data being sent is a `FormData` object, and instead, remove any content-type header so that a proper one containing the boundary will be added by the dart's internal plumbing.