add column_type method to Models

sometimes its useful for creating reusable code.

It used to be available as an undocumented method in rails FYI...

module ActiveRecord 
  if RUBY_ENGINE == 'opal'
    class Base
      # datetime, date, boolean, float, integer, string
      def column_type(attr)
        backing_record.column_type(attr)
      end
      def self.column_type(attr)
        ReactiveRecord::Base.column_type(columns_hash[attr])
      end
    end
  end
end