edd_ajax_apply_discount fix

When calling 'edd_ajax_apply_discount' via ajax, response fails with the following messages:

Notice:  Undefined variable: key in ..\easy-digital-downloads\includes\discount-functions.php on line 761
Notice:  Undefined variable: value in ...\easy-digital-downloads\includes\discount-functions.php on line 762

In ajax-function.php on line 164 there is a call to <code>edd_is_discount_valid()</code> without passing a user. That function later calls <code>edd_is_discount_used()</code> which failed because no user is sent.

P.S. Sorry for not creating issue first, I just read the guidelines, next time I will ;)