Fixing deprecation warning for nil columns in Rails 4.2.beta1

This is an alternative solution for outstanding pull request #1094 

The underlying issue is that simple_form calls `column_for_attribute` on non-attribute inputs, for instance, password on a devise model. This results in a deprecation message from rails. The fix uses `has_attribute?`
to ensure we don't call `column_for_attribute` improperly.