Prevent duplicate selectors being added to reset ruleset

[simonsmith]

If a selector appeared more than once (media query, nesting) it was being duplicated in the reset ruleset. This change adds a check for the selector first.

cc @giuseppeg

[giuseppeg]

Since we are in ES6 I would use a Set instead of an Array – access with has is O(1) and you won't need the extra contains helper

[simonsmith]

That's a good shout but I'd need to add the babel polyfill and I wonder if it's worth the extra code.

[giuseppeg]

@simonsmith ok, your call! the changes looks good to me

[maximkoretskiy]

@simonsmith Could you add changes description into unrealesed section of changelog

[simonsmith]

@maximkoretskiy Done

[simonsmith]

I'll just rebase this with your latest changes

[maximkoretskiy]

@simonsmith sorry. Just pushed fix. could you resolve the conflict?

[maximkoretskiy]

Just published your changes to npm. Thank U guys!