Return a 204 (Codes::HTTP_NO_CONTENT) when content data is provided.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.5
http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.5
made status code configurable when view data is explicitly null
made serializing null values configurable
some changes
Merge pull request #356 from j/empty-responses

Empty responses