Charge API: charge_token in error response?

[Ross Poulton]

Via @huwshimi:

Also in 'process_transaction()' if the response has an error there is check for "self.transaction_token = r['charge_token']". I could not reproduce a response including 'charge_token' but I may have not make the right request. I added 'charge_token' to the mocked request so that the tests pass, but wanted to check this was correct.

I'm unsure why this is in there; need to validate that it's part of the Charge API when an error occurs.