Fix Context#find for Hash-like objects

This removes the exclusion of `Hash`-like objects from method execution through `Context#find`.

I encountered this problem while trying to get the `size` of a object inheriting from `Hash`.

I usually would not check for `Hash` at all but this would clash with use cases like [`test_id_with_nested_context`](https://github.com/defunkt/mustache/blob/master/test/mustache_test.rb#L495-508) where you wouldn't be able to access the `:id` key because of `Object#id`.
A possible solution for that problem would be a special `Hash` subclass for the Mustache contexts. Any thoughts on that?