Cancellation of requests with (partial) responses

[martinthomson]

We should be a lot clearer about what it means to receive HTTP_REQUEST_CANCELLED for a stream where part of the response was received. What if you receive (in the same packet even) a STREAM frame containing the headers block and a 200 status code, followed by a cancellation of that stream?

This also applies to GOAWAY, which implicitly sends HTTP_REQUEST_CANCELLED for all requests with stream ID > N.

[MikeBishop]

#770 appears to cover this as well, though the part about CANCELLED should perhaps move elsewhere in the doc.