Load columns from schema to avoid AR ignored/only column configuration

[bensheldon]

Fixes #286

This PR recreates the Active Record logic for fetching the columns from the schema cache before they are filter based on Active Record's ignored_columns/only_columns configuration.

• columns is defined here: https://github.com/rails/rails/blob/5d4cfc3e73a77bd5ee4fba3ec42e8c3782c9ab02/activerecord/lib/active_record/model_schema.rb#L443-L450
• ...and populated here: https://github.com/rails/rails/blob/5d4cfc3e73a77bd5ee4fba3ec42e8c3782c9ab02/activerecord/lib/active_record/model_schema.rb#L609-L615