Revisit making Response.BodyPipe.Complete() actually complete the PipeWriter for Http2

[jkotalik]

In the Http1 and Http2 output producers, we currently make Complete effectively noop. For Http1 we can't call PipeWriter.Complete() as that is the connection pipe and there isn't a notion of a response pipe. For Http2, we CAN call Complete() as there is a Data pipe available. However, there are issue with creating a well formatted response if we do support Complete.

• Do we require users to call StartAsync() before calling Complete(). This will guarantee that headers are committed.
• How do we handle Trailers? ProduceEnd?

[Tratcher]

This would prevent middleware from adding trailers.

[Tratcher]

Isn't this superseded by CompleteAsync?