Fix to erronous type comparison in scrollfix module

On `Google Chrome 28.0.1500.68` the scrollfix module causes the following error:

    TypeError: Object 165 has no method 'charAt'
        at link (http://localhost:8080/components/angular-ui-utils/modules/scrollfix/scrollfix.js:18:31)
        at nodeLinkFn (http://localhost:8080/components/angular-unstable/angular.js:4959:13)
        at compositeLinkFn (http://localhost:8080/components/angular-unstable/angular.js:4550:15)
        at compositeLinkFn (http://localhost:8080/components/angular-unstable/angular.js:4553:13)
        at compositeLinkFn (http://localhost:8080/components/angular-unstable/angular.js:4553:13)
        at nodeLinkFn (http://localhost:8080/components/angular-unstable/angular.js:4953:24)
        at compositeLinkFn (http://localhost:8080/components/angular-unstable/angular.js:4550:15)
        at nodeLinkFn (http://localhost:8080/components/angular-unstable/angular.js:4953:24)
        at http://localhost:8080/components/angular-unstable/angular.js:5099:15
        at nodeLinkFn (http://localhost:8080/components/angular-unstable/angular.js:4953:24) <p ui-scrollfix=""> angular.js:6349

`angular-unstable` is version `1.1.5` from [Johannes Troeger's repository](https://github.com/johannestroeger/bower-angular-unstable).

I added type detection to avoid running string operations on incompatible data types.