Fix for invalid merchantSig param

I've had some cases where Adyen redirects back (after a payment) with the following param string:

    ?merchantReference=someref&skinCode=somecode&shopperLocale=en&paymentMethod=payment_method&authResult=AUTHORISED&pspReference=123123123123&merchantSig=19TtXQrl/Y61HJW8+Qd/PO5Gwms=

The important part here is the merchantSig containing '+'. Rails (v3.2.16) turns this into "19TtXQrl/Y61HJW8 Qd/PO5Gwms=" (replacing the '+' with a space).

As I presume the merchantSig does not contain spaces we just replace them.