Headers applied by HttpDefaultHeaders service breaks multipart content requests.

[bgourlie]

This bug emerged from a fix for this bug.

When passing FormData to the sendData param of an http request, some internal dart plumbing will add a content-type header that looks like this:

Content-Type: multipart/form-content; boundary=----WebKitFormBoundary5b...

The boundary is used by the server to delimit the different parts of the request body. If I manually specify the Content-Type header, the boundary will not be included, and if I omit it altogether, angular will add a default content-type (JSON). Either way, it prevents multipart requests from being properly handled by the server.