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
I don't think that it's a good idea because before the `if` had no `else` therefore the comment is above the statement.

Now the `if`have an `else`therefore 2 comments : one for the if (the same than the previous version) and one for the else.