Deal with to_param CPK IDs in one place. Fixes #209.

We parse the IDs in the first line of the finder, and isolate the parsing to one method in case it needs to be used elsewhere (and also so it doesn't make the finder method hard to read).

I added 3 tests to check that the parsing works, and all the other tests still pass (on mysql, postgresql, and sqlite3) so it did not affect the passing in of arrays.

I feel like the code for the parsing could be better, but I'm failing at finding specific ideas to improve it.

Do you seen any deficiencies?