fix case sensitive apply coupon mutation (#729)

The applyCoupon mutation is case sensitive, but coupons are not. This fixes trying to add a coupon code with lower case letters, such as `10off`. Previously you would get an error that the coupon doesn't exist because of mismatch in the case.

Author: sbolinger-godaddy

includes/data/mutation/class-cart-mutation.php

@@ -170,7 +170,7 @@ public static function validate_coupon( $code, &$reason = '' ) {
 		$the_coupon = new \WC_Coupon( $code );
 
 		// Prevent adding coupons by post ID.
-		if ( $the_coupon->get_code() !== $code ) {
+		if ( strtoupper( $the_coupon->get_code() ) !== strtoupper( $code ) ) {
 			$reason = __( 'No coupon found with the code provided', 'wp-graphql-woocommerce' );
 			return false;
 		}