Implements `Object#in?`

I ran into the same error that you did, the weird 'specs' bug.  I got around it by using an alias.

I implemented the *original* version of `in?`, see these conversations:

<https://github.com/rails/rails/pull/258>
<https://github.com/rubymotion/sugarcube/issues/76>
<https://github.com/rails/rails/pull/10394>