test case to prevent duplicated associations with custom PK.

This is only a test-case to prevent regressions for the issue #9201. I think we should include it in rails master where it's already fixed to make sure we don't run into the same problem again.

I would be willing to backport the fix to 3-2-stable but I have no idea what commit actually fixed it. I tried to bisect but was with no luck because I could not get bundle to work on most commits (renamed gems, missing gems, etc...).