Checkout attribute validation

[jithendraprasad9999]

Resolves #303
Type: bugfix

Issue

In shopping cart page when we working as guest even though checkout attribute pre-selected value is none, it is defaulting to the first value,Even though it is a mandatory field, it is allowing us to proceed without any selection.

Solution

1.Added additional API call on "Continue As Guest" button to validate the checkout attribute using existing API from checkout controller. 2.And unified checkout buttons code

Breaking changes

none

Testing

1. Navigate to shopping cart without any user login .
2. If we tap on "Checkout As Guest" button without selecting any checkout attribute which is mandatory . One should get a model state error