[whatwg/dom] Unify behavior for error handling in supports() and contains() methods (#202)

Both this  methods don't modificate tokens so probably should have similar behavior when `token` argument has empty string and ASCII whitespace. Currently `contains()` throw (like all other DOMTokenList methods) and `supports()` doesn't throw. So there is two solution:
- change `supports()` to throwing, this method is new so compatibility should not be a problem.
- change `contains()` to not throwing because in this scenerio it's unnecessary (we don't change tokens) so we get little simplification.

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