Coupon codes entered in a case different than the stored code are not caught as already applied

Problem/Motivation

Create a Coupon code "WHOOPS".

Apply the coupon using "WHOOPS". It works.

Apply it again, using "WHOOPS". It reports that the coupon has already been applied to the order.

Apply it again using "whoops".

Because the comparison is using the == operator a case-sensitive comparison is done and the coupon is not rejected. You can repeatedly apply the coupon. For a fixed price coupon, this can drive the price to 0 and below!

Proposed resolution

Use strcasecmp to do a case insensitive comparison. My only concern is most of the other comparisons are done using database queries, so if the backend database uses a different algorithm, we could end up with different results in other code paths.

Remaining tasks

Patch will be added after I post this and get an issue number. Just a one line change.