The `to_dict` method on `Entity` was not looking for columns in the case of multi-table inheritance.  Now, the columns to be included in the to_dict are copied from the `mapper.tables` list.
The `to_dict` method on `Entity` was not looking for columns in the case of multi-table inheritance.  Now, the columns to be included in the to_dict are copied from the `mapper.tables` list.

gjhiggins/elixir