Improve json_response to allow adjusting the HTTP code.
Improve json_response to allow adjusting the HTTP code.

Conservatively utilizes the new json_response behavior. For apis which claim
to respond with JSON, we should always respond with JSON. However, I know that
some of the non-JSON responses may be expected by clients, so I don't correct
everything that I could correct.

R=maruel@chromium.org
BUG=

Review URL: https://codereview.appspot.com/17890043/

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