Dont set content-length header on streamed response. Fixes #13991

Thanks to @AsgerPetersen to help fixing #13991
closes #2582

Author: rldhont

src/server/qgshttprequesthandler.cpp

@@ -461,6 +461,7 @@ bool QgsHttpRequestHandler::startGetFeatureResponse( QByteArray* ba, const QStri
     format = "text/xml";
 
   setInfoFormat( format );
+  sendHeaders();
   appendBody( *ba );
   // Streaming
   sendResponse();