scroll-to does not work with a custom scrollable which is at the top of the document

[chbonser]

The fundamental problem is the addon recommends setting the scrollable property on the service. This leaves the service to perform an inaccurate check to see if the user overwrote the scrollable propery (https://github.com/c4eo/ember-scroll-to/blob/master/addon/services/scroller.js#L39). Clearly this check doesn't work if top is 0.

I'll submit a PR shortly which adds an acceptance test and fixes the bug. I won't change the recommendation to set scrollable as that would be a breaking change.