Fix index detection for HABTM invoving an STI subclass

[RST-J]

This (should) fix(es) #67 plus becomes precise about expected warnings/errors.

[plentz]

@RST-J have you run the tests using rake? this way, it will test(and update) all scenarios that we support :)

[RST-J]

Yes, I used rake spec but I should have used appraisal rake spec. Considering my comment in the code above, maybe it would be a good idea to have different setups for different rails version - though I must admit I don't know how or even whether this is possible.