Update the set_serializer test to match the actual output.

tests/test_set_serializer.expected

@@ -1,3 +1,4 @@
+Test setting, then resetting a custom serializer:
 my_object.to_string(standard)={ "abc": 12, "foo": "bar" }
 my_object.to_string(custom serializer)=Custom Output
 Next line of output should be from the custom freeit function: