gRPC: special handling of "trailer only" responses. The gRPC protocol makes a distinction between HEADERS frame with the END_STREAM flag set, and a HEADERS frame followed by an empty DATA frame with the END_STREAM flag. The latter is not permitted, and results in errors not being propagated through nginx. Instead, gRPC clients complain that "server closed the stream without sending trailers" (seen in grpc-go) or "13: Received RST_STREAM with error code 2" (seen in grpc-c). To fix this, nginx now returns HEADERS with the END_STREAM flag if the response length is known to be 0, and we are not expecting any trailer headers to be added. And the response length is explicitly set to 0 in the gRPC proxy if we see initial HEADERS frame with the END_STREAM flag set.