(WIP) Retrieving active coupon codes from model

@cdodge I was looking into the code and came to know that the model [shoppingcart_coupon] can contains multiple INACTIVE coupon_codes  but only one ACTIVE coupon_code. 

Testing Scenario: 
- Add a coupon code "abcde" . 
- Delete it,  implementation will mark is_active = false for that coupon code. (still exist in database). 
- User add the same coupon code "abcde" .  [Now two same code but only one is active]

So I am updating the query in which i am getting the ACTIVE coupon code only from model. 

another thing, i do not not know why automated test cases are going to fail , I think u mentioned that issue in email.

Kindly review the change set.
 
Thanks.