This patch is extending the Response object and the Pecan extension
to make it possible to return a non-default return type (just like it
already supports retuning a non default status code).
This is useful when the return code of a function is 204 (No Response) for
that case we automatically set the return_type to be None and guarantee
that the response body is empty and the content-lentgh is 0. Which if
not done can cause tools consuming the API to misbehave.
Change-Id: I629c3e2d5dc169154e5af044f1b8f804e4b87e23
de877d2cc8