Use RST_STREAM(NO_ERROR) in case server early respond (#633) (#634)

Http2 Server are allowed to early respond without fully
  consuming client input stream, but must respond with an
  error code of NO_ERROR when sending RST_STREAM.
  Nginx treat any other error code as fatal if not done so

  Commit change error code from CANCEL to NO_ERROR, when the
  server is early responding to the client

  https://github.com/hyperium/h2/issues/633
  https://trac.nginx.org/nginx/ticket/2376

tests/h2-tests/tests/server.rs

@@ -566,7 +566,9 @@ async fn sends_reset_cancel_when_req_body_is_dropped() {
         client
             .recv_frame(frames::headers(1).response(200).eos())
             .await;
-        client.recv_frame(frames::reset(1).cancel()).await;
+        client
+            .recv_frame(frames::reset(1).reason(Reason::NO_ERROR))
+            .await;
     };
 
     let srv = async move {