nav2_costmap_2d: add missing default_value_ copy in Costmap2D operator= (ros-navigation#4753)

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

Signed-off-by: Dylan De Coeyer <dylan.decoeyer@quimesis.be>
Signed-off-by: Jakubach <jakubach@gmail.com>

Author: DylanDeCoeyer-Quimesis

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_);