(PUP-1549) Use standard reason phrases on webrick

The webrick interface was using the body of the response for the reason
phrase for non-2xx responses. In the v1 api, this meant that we got
bizarre response headers. In the v2 api, it meant that we got json
formatted body data in the first header.

Not only was that state of affairs odd, it was also inconsistent with the
rack implementation, which didn't every set the reason phrase. I am also a
little concerned that some clients might have choked on large reason
phrases (header too long issues).

This removes the setting of the reason phrase on webrick, which give us
back the standard reason phrases.