In line with ngSrc and ngHref, this new directive ensures that the
`srcset` HTML5 attribute does not include a pre-interpolated string.
Without it the browser will fetch from the URL with the literal text
`{{hash}}` until AngularJS replaces the expression inside `{{hash}}`.
Closes#2601
IE9 ignores setAttribute('src', val) calls on img if "ng:src" attribute
is present. It only fetches the image if element property is updated as well.
Closes#935