Make default double serializer ignore userdata again

The user might want to use the userdata for something different, so the
serializer should ignore it by default.

Explicitly setting the serializer to json_object_double_to_json_string will
still make it interpret the userdata as a format string.

tests/test_double_serializer.expected

@@ -0,0 +1,8 @@
+Test default serializer:
+obj.to_string(standard)=0.5
+Test default serializer with custom userdata:
+obj.to_string(userdata)=0.5
+Test explicit serializer with custom userdata:
+obj.to_string(custom)=test
+Test reset serializer:
+obj.to_string(reset)=0.5