Pass the AccessCode from previous response instead of trying to split on = in Location header

@ntalbott @melari 

eWAY Rapid AccessCode can be padded with = (equals) and I suspect contain them too. This causes issues as it passes an incorrect AccessCode to `run_purchase` for a select few merchants.

Instead of trying to parse the AccessCode from the Location header, is there anything wrong with simply passing it from the previous request?