Add #key? to ActiveModel::Errors

Mirror Ruby's [`Hash#key?`](http://ruby-doc.org/core-2.1.3/Hash.html#method-i-key-3F)
So that, I think, it will have a similar entry in the documentation to [`has_key?`](http://api.rubyonrails.org/classes/ActiveModel/Errors.html#method-i-has_key-3F).