Servers should send RST_STREAM(NO_ERROR) after trailers if client is not half-closed

The HTTP/2 spec [calls out this case specifically](http://httpwg.org/specs/rfc7540.html#rfc.section.8.1) 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.