[QGIS-Server][BUGFIX] GetLegendGraphics set layername

To have title instead of name in server getlegendgraphic response, the commit
 bb95a0f update getlegendgraphic to set the
 tree layer name. This update set the layer name with layer title.

To fix it, I just store the layers name and reset them at the end of
 GetLegendGraphic request.

src/mapserver/qgswmsserver.cpp

@@ -656,11 +656,20 @@ QImage* QgsWMSServer::getLegendGraphics()
     }
   }
 
+  // Create the layer tree root
   QgsLayerTreeGroup rootGroup;
+  // Store layers' name to reset them
+  QMap<QString, QString> layerNameMap;
+  // Create tree layer node for each layer
   foreach ( QString layerId, layerIds )
   {
+    // get layer
     QgsMapLayer *ml = QgsMapLayerRegistry::instance()->mapLayer( layerId );
+    // create tree layer node
     QgsLayerTreeLayer *layer = rootGroup.addLayer( ml );
+    // store the layer's name
+    layerNameMap.insert( layerId, ml->name() );
+    // set layer name with layer's title to have it in legend
     if ( !ml->title().isEmpty() )
       layer->setLayerName( ml->title() );
   }
@@ -792,6 +801,13 @@ QImage* QgsWMSServer::getLegendGraphics()
 
   p.end();
 
+  // reset layers' name
+  foreach ( QString layerId, layerIds )
+  {
+    QgsMapLayer *ml = QgsMapLayerRegistry::instance()->mapLayer( layerId );
+    ml->setLayerName( layerNameMap[ layerId ] );
+  }
+  //  clear map layer registry
   QgsMapLayerRegistry::instance()->removeAllMapLayers();
   return paintImage;
 }