Fix 202 responses to contain valid content.

Using a webob.exc to generate 202s makes the response return plain
text in the body, which breaks things that expect to be able to
parse data according to the content type. Using a webob.Response
object makes sure the body of the 202 response is empty.

Change-Id: Ifca3081f573407808a2196e71ac1db87180db6ec
Resolves-bug: #1356058
Merge "Fix 202 responses to contain valid content."