Clean up some of the proxy unit tests.

Since we've got machinery to catch HTTPException errors that happens
after the proxy server's __call__ method but before the individual
GET()/POST()/etc. methods, we shouldn't just call
GET()/POST()/etc. Instead, we should use swob.Request's get_response()
method, or otherwise actually call the proxy application. That way we
can have the code raise HTTPBadRequest, but the test just gets a
plain old response with a 400 status code, just like you'd expect.

Earlier changes fixed some of the tests; this commit fixes some
more. In the interest of not having a gigantic diff, this only gets
about half the proxy's unit tests, and the others will be done in a
subsequent commit.

Change-Id: Iaca32e74a8f4499440e655c207a1012e14486c74
Merge "Clean up some of the proxy unit tests."