Fix #342 without expanding API

@AntJanus,
when data-src is dynamically updated, angular is updating the src attribute value

So setting data-src value with src value is fixing #342 without having to introduce a new attribute.
I tested the below example with this fix and it is working fine.

```html
<ul ng-init="items=['assets/img/iconic/action.svg', 'assets/img/iconic/ban.svg', 'assets/img/iconic/bell.svg']">
  <li ng-repeat="item in items" >
  <img zf-iconic="" data-src={{item}} class="iconic-sm" >
  </li >
</ul>
```