Refactor Stripe::Util.convert_to_stripe_object method

The `Hash#fetch` method neatly abstracts the "lookup this key and if it's not in the hash use this default value" pattern. Jumped out at me whilst reading through the code.

Also: the comment doesn't agree with the implementation: it suggests the default value should be `APIResource` instead of `StripeObject`. Not sure which is correct, but one of them presumably needs changing.

#NotUrgentButEveryLittleHelps :)