Support HTTP_1_1_REQUIRED on the server

This is needed to support draft15 fully. This is the server-side only implementation (in other words, adding HTTP_1_1_REQUIRED to our list of known error codes). The client-side implementation is more complex, and (perhaps) less important, given that it's really only necessary to support TLS certificate auth. (Though why you would send this error code and not just *not* advertise h2 for those servers is kind of beyond me... MS seemed to convince everyone, though). Either way, the client-side implementation is not nearly as important, and can wait :)