Fix Decorator#inspect on Ruby 1.9

As [pointed out](https://github.com/drapergem/draper/issues/560#issuecomment-20579840), Ruby 1.9 uses `to_s`, if defined, for `inspect`, so with `to_s` delegated (#561), it's difficult to distinguish the decorator from the object at the console.

This hacks `inspect` to return the same thing on 1.9 as it does on 2.0