Fix add_child on Marker not returning self

Author: hansthen

folium/map.py

@@ -445,6 +445,7 @@ def add_child(self, child, name=None, index=None):
             self.set_icon(child)
         else:
             super().add_child(child, name, index)
+        return self
 
 
 class Popup(MacroElement):