fix #2

`scope.active` is `undefined` by default, see [here](https://github.com/turn/infinite-scroll/blob/master/src/infinite-scroll.coffee#L27)

The user can then use it to programmatically activate/deactivate infinite-scroll, see [here](https://github.com/turn/infinite-scroll/blob/master/src/infinite-scroll.coffee#L109).

The issue was the when `active` isn't set by the user, it is falsey, so #check will always return false. We only want it to return false when the user explicitly set it to false.