nghttpx: Fix bug that proxy with HTTP/1.1 CONNECT did not work

This was a regression in 5fbe4cc225.

src/shrpx_http2_downstream_connection.cc

@@ -108,7 +108,9 @@ int Http2DownstreamConnection::attach_downstream(Downstream *downstream) {
   auto &req = downstream_->request();
 
   // HTTP/2 disables HTTP Upgrade.
+  if (req.method != HTTP_CONNECT) {
     req.upgrade_request = false;
+  }
 
   return 0;
 }