Client should verify response content-type

[jhump]

Currently, the client assumes the response uses the correct content-type if it has a "200 OK" status code. To prevent strange issues in the face of misbehaving servers or middle-boxes, the client should actually verify the content-type.

If the content-type does not appear to be a valid RPC response (i.e. incorrect prefix), it should consider it an unknown error. If the content-type does appear to be a valid RPC response (correct prefix) but indicates the wrong codec (i.e. different than the request codec), it should consider it an internal error (since this indicates an internal problem in the server implementation).