drnic/composite_primary_keys — pull request #95 by kuroda (2012-03-27 00:23:00+00:00)
Title: to_param should join with comma instead of minus sign

This patch overrides `to_param` of ActiveModel::Conversion, which joins ids with minus sign.

I wrote a test for this patch, but I am not sure if the position is correct and I wrote it in a suitable way.

Please instruct me if I am wrong.