make sure all layer masters are case similar

nav2_costmap_2d/src/costmap_2d_publisher.cpp

@@ -227,7 +227,7 @@ Costmap2DPublisher::costmap_service_callback(
   response->map.metadata.size_x = size_x;
   response->map.metadata.size_y = size_y;
   response->map.metadata.resolution = costmap_->getResolution();
-  response->map.metadata.layer = "Master";
+  response->map.metadata.layer = "master";
   response->map.metadata.map_load_time = current_time;
   response->map.metadata.update_time = current_time;
   response->map.metadata.origin.position.x = costmap_->getOriginX();

nav2_util/src/costmap.cpp

@@ -87,7 +87,7 @@ void Costmap::set_test_costmap(const TestCostmap & testCostmapType)
 {
   costmap_properties_.map_load_time = node_->now();
   costmap_properties_.update_time = node_->now();
-  costmap_properties_.layer = "Master";
+  costmap_properties_.layer = "master";
   costmap_properties_.resolution = 1;
   costmap_properties_.size_x = 10;
   costmap_properties_.size_y = 10;