Verify that server sends RST_STREAM(NO_ERROR) after trailers if client is not half-closed

[ejona86]

The HTTP/2 spec calls out this case specifically saying that servers MAY send the RST_STREAM. For gRPC, we are going to consider it more of a SHOULD, although clients still have to assume it is a MAY because intermediaries may in be play.

This was a bug in the Java server (grpc/grpc-java#2162). I have no clue how Go currently behaves in this case.