Fix OAuth1 to not JSON-encode create access token response

The OAuth1 response was a JSON-encoded urlencoded string rather
than just a urlencoded string as expected.

This also enhances keystone.middleware.wsgi.render_response() to
support setting the Content-Type header in the response.

Closes-Bug: #1336910

Change-Id: I72dcb61cae712c3c76382d2d6067b4f5fad7729d
Merge "Fix OAuth1 to not JSON-encode create access token response"