Refactor Stripe::Util.convert_to_stripe_object method

* Extract hash mapping into object_classes method/ivar
* Use Hash#fetch instead of `if` and `||=`
Fix comment and use two argument form of Hash#fetch
Merge pull request #73 from timcraft/refactor-util-convert_to_stripe_object

Refactor Stripe::Util.convert_to_stripe_object method