Change RequestFactory based requests:
- No longer require a 'Content-Length' HTTP request header, thus enabling support for 'Transfer-Encoding: chunked'
- Use 'application/json; charset=utf-8' Content-Type instead of 'text/plain; charset=utf=8'

Review at http://gwt-code-reviews.appspot.com/674804

Review by: amitmanjhi@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8466 8db76d5a-ed1c-0410-87a9-c151d255dfc7