fix build break

Change-Id: I8d254d16c9b95fc43bf894b0497edb9a366ee0d2
Fix up explicit setting of the Content-Type.
This is handled automatically by AsyncHttpResponseImpl.

Behavior changed that the AsyncHttpRequest no longer adds default headers. Accomodate.

Change-Id: I83e64fed79e70622ddc9171591f1f54213c20a31