`destroy()` shouldn't remove the `sticky-native` class

If the purpose of `destroy()` is to remove the polyfill’s capabilities,
it doesn’t need to remove the `sticky-native` class, which could still
be relied on by the stylesheet.
the `polyfill-active` class now spots if top:0 is ok

Some browsers need the sticky element to have a `top:X` or `bottom:X`,
this now determines if `polyfill-active` is added to the document or
not.
Merge branch 'master' into active/native-classes