fix XSS vulnerability in ng-srcset

There is a security issue with `ng-srcset`.

**URIs aren't sanitized**.

The behiavor is correct for `ng-src` but not for `ng-srcset`.

The probleme is with `src/ng/compile.js` that ignore `srcset`.

Here is the fix.

Laurent Trillaud
Sorry for that. It's my first pull request!