Fix Decorator#inspect on Ruby 1.9

1.9 uses `to_s`, if defined, for `inspect`, so with `to_s` delegated,
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
Merge pull request #563 from haines/to_s

Fix Decorator#inspect on Ruby 1.9