nghttpx: Fix bug that zero-length POST is not forwarded

src/shrpx_http_downstream_connection.cc

@@ -561,6 +561,8 @@ int HttpDownstreamConnection::push_upload_data_chunk(const uint8_t *data,
 }
 
 int HttpDownstreamConnection::end_upload_data() {
+  signal_write();
+
   if (!downstream_->get_chunked_request()) {
     return 0;
   }
@@ -577,8 +579,6 @@ int HttpDownstreamConnection::end_upload_data() {
     output->append("\r\n");
   }
 
-  signal_write();
-
   return 0;
 }