In json_object_deep_copy(), copy over _userdata, at least for json_type_string's with the default serializer set, and provide a way for people using custom serializers to provide a custom shallow_copy method.

tests/test_deep_copy.expected

@@ -32,7 +32,8 @@ Printing JSON objects for visual inspection
       }
     }
   }
-}------------------------------------------------
+}
+------------------------------------------------
 ------------------------------------------------
  JSON2
 {
@@ -112,7 +113,8 @@ Printing JSON objects for visual inspection
       }
     ]
   }
-}------------------------------------------------
+}
+------------------------------------------------
 ------------------------------------------------
  JSON3
 ------------------------------------------------
@@ -137,4 +139,10 @@ Printing JSON objects for visual inspection
       ]
     }
   }
-}------------------------------------------------
+}
+------------------------------------------------
+
+Testing deep_copy with a custom serializer set
+CALLED: my_shallow_copy on with_serializer object
+
+deep_copy with custom serializer worked OK.