Added test for posting null entity with content-type.

- Verifies that posting an empty request with a content-type header works (based on a user's list discussion).

Change-Id: Ib70bd6a5cfc8bdec224d5eae69d439c75947737b
Signed-off-by: Marek Potociar <marek.potociar@oracle.com>
Merge "Added test for posting null entity with content-type."