[Bugfix] Payment Token metadata hash should be with indifferent access

**Problem** 
If a `Billing::PaymentToken` is initialized with an `ActiveSupport::HashWithIndifferentAccess`, you cannot `symbolize_keys!` ...[because it's peculiarly absent from that class.](http://api.rubyonrails.org/classes/ActiveSupport/HashWithIndifferentAccess.html)

**Solution** 
For the Payment Token `@metadata` hash to be initialized with indifferent access. 
(Bonus: any subclass won't need to worry about stringifying or symbolizing, which is probably the way it should've been from the start.)

Please review @girasquid @edward