Better method signatures for Routing.

Also, addExtensions() already array_unique()s it. No need to do that twice then.

Refs https://github.com/cakephp/cakephp/pull/3802
I agree, the current method signatures could easily be unified with others where we do the same thing, making the additional addExtensions() method unnecessary.

PS: @markstory was right, using extensions($notEmptyValue) we would actually end up with dupes here with the current code. But it should array_unique on write, not read IMO.