Add serialized form data to ajax call

Hi Pippin,

Small PR, this adds the fields of the purchase form to the AJAX call to validate the coupon when pressing 'apply'.

In my scenario I want to validate the user email; which is possible when someone is logged in, but when a guest wants to checkout I want to verify the checkout email. 

The code is pretty global and can also be used for other purposes.

Let me know what you think.

Thanks and happy new year :fireworks:,
Jeroen

PS. what do you use for minification/uglification of JS?