Fix #217 SkinnyRecord#save() with TimestampsFeature doesn't update updatedAtField

This is a general solution for #217. 

If skinny detects `#skinnyCRUDMapper` is a `TimestampsFeature` wired one (via Java reflection API) and automatically filters timestamps for library users, it may be handy.

However, I'm not willing to do that because of the following reasons:

- the implementation won't be type-safe
- the mechanism is a little bit black magic