Mail: fixed SMTP pipelining to send the response immediately. Previously, if there were some pipelined SMTP data in the buffer when a proxied connection with the backend was established, nginx called ngx_mail_proxy_handler() to send these data, and not tried to send the response to the last command. In most cases, this response was later sent along with the response to the pipelined command, but if for some reason client decides to wait for the response before finishing the next command this might result in a connection hang. Fix is to always call ngx_mail_proxy_handler() to send the response, and additionally post an event to send the pipelined data if needed.

scgi_param  REQUEST_METHOD     $request_method;
scgi_param  REQUEST_URI        $request_uri;
scgi_param  QUERY_STRING       $query_string;
scgi_param  CONTENT_TYPE       $content_type;

scgi_param  DOCUMENT_URI       $document_uri;
scgi_param  DOCUMENT_ROOT      $document_root;
scgi_param  SCGI               1;
scgi_param  SERVER_PROTOCOL    $server_protocol;
scgi_param  REQUEST_SCHEME     $scheme;
scgi_param  HTTPS              $https if_not_empty;

scgi_param  REMOTE_ADDR        $remote_addr;
scgi_param  REMOTE_PORT        $remote_port;
scgi_param  SERVER_PORT        $server_port;
scgi_param  SERVER_NAME        $server_name;