nav2_costmap_2d: add missing default_value_ copy in Costmap2D operator=

default_value_ is an attribute of the Costmap2D class and should be
copied along with the other attributes.

nav2_costmap_2d/src/costmap_2d.cpp

@@ -219,6 +219,7 @@ Costmap2D & Costmap2D::operator=(const Costmap2D & map)
   resolution_ = map.resolution_;
   origin_x_ = map.origin_x_;
   origin_y_ = map.origin_y_;
+  default_value_ = map.default_value_;
 
   // initialize our various maps
   initMaps(size_x_, size_y_);