Merge pull request #5704 from jtattermusch/fix_python_beta_stream_stream

Fix race between send message and send initial metadata

src/core/transport/chttp2_transport.c

@@ -851,9 +851,11 @@ static void perform_stream_op_locked(
     if (stream_global->write_closed) {
       grpc_chttp2_complete_closure_step(
           exec_ctx, &stream_global->send_message_finished, 0);
-    } else if (stream_global->id != 0) {
+    } else {
       stream_global->send_message = op->send_message;
-      grpc_chttp2_become_writable(transport_global, stream_global);
+      if (stream_global->id != 0) {
+        grpc_chttp2_become_writable(transport_global, stream_global);
+      }
     }
   }