Upstream: ngx_post_event() instead of upgraded call (ticket #503). If a request is finalized in the first call to the ngx_http_upstream_process_upgraded() function, e.g., because upstream server closed the connection for some reason, in the second call the u->peer.connection pointer will be null, resulting in segmentation fault. Fix is to avoid second direct call, and post event instead. This ensures that ngx_http_upstream_process_upgraded() won't be called again if a request is finalized.