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
edd_is_discount_valid() without passing a user. That function later
calls edd_is_discount_used() which failed because no user is sent.
Merge pull request #2346 from somewebmedia/master

edd_ajax_apply_discount fix