Failing test for ActiveModel::Errors#has_key? return

From the [doc](http://api.rubyonrails.org/classes/ActiveModel/Errors.html#method-i-has_key-3F), this method should return `false`and not `nil` if there is no errors for this key

I thinks it's logical to return `false` also because the method finish with an `?`.
If it's not the case, I can still update the doc instead.