Simple fix for payment source.

Steps to reproduce the issue:

1. Add an order through the admin interface
2. Attempt to pay via cheque for a user who has a credit card number
stored in their profile.
3. Submit the form

This will 500, because params[:card] is set from the form on the initial
page for the radio button from the credit card profiles.

This is a simple, though incomplete fix for the issue. A proper fix
would be to change the form so that params[:card] is qualified by the
payment_method which it belongs.

There are also likely issues with multiple payment methods which support
profiles, as there is likely to be confusion between them. This should
suffice for now.