Fix POST handling without data + Fix HEAD which should return no body

Summary: This PR does two things:

 - It fixes the POST handling when there is no data (#3234).
 - It fixes the HEAD handling which should return no body.

After looking to the tests, I don't know how to do it :s

Closes https://github.com/facebook/hhvm/pull/3240

Reviewed By: @ptarjan

Differential Revision: D1446409