[BUGFIX][QGIS Server] Clean Headers

With https://github.com/3liz/qgis-wfsOutputExtension, I found that QGIS Server
 Headers are not always well formed. An extra line can be added at the end.

In the commit a9c830e from @mhugent, I found
 the fix.

src/server/qgshttprequesthandler.cpp

@@ -131,7 +131,6 @@ void QgsHttpRequestHandler::sendHeaders()
       printf( it.value().toLocal8Bit() );
       printf( "\n" );
     }
-    printf( "\n" );
   }
   printf( "\n" );
   mHeaders.clear();