Fix for #668 - Default header template ngSorted style should only be applied during an active sort

The issue is located in headerCell Template, line 1:

ng-class="{ 'ngSorted': !noSortVisible }"

This is only testing for the existence of the noSortVisible function, not actually invoking the method.  Pull request correctly invokes the method.

BEFORE: http://plnkr.co/edit/IVbkGFAKGATKgyitdrrD?p=preview

AFTER: http://plnkr.co/edit/ZbRQZHTL0XMx1jyShdb1?p=preview