Simplify and test twilify and detwilify

This does a few things:

* Removes tests for the `Twilio::REST::Utils#twilify` method from the client spec
* Creates a spec for `Twilio::REST::Utils` and adds the `twilify` test
* Adds tests for `twilify` and `detwilify`
* Simplifies and removes duplication from the `twilify`/`detwilify` methods