Foreign keys can be symbols for has_and_belongs_to_many

Fixes https://github.com/thoughtbot/shoulda-matchers/issues/583
Yes, they were. The test setup supplies the `custom_foreign_key_id` column as a symbol, so it should not be flagged as missing in the error message. Only the `relative_id` should. Because of #583 the column was not being recognised previously and the error message that the test was expecting was actually a bug as it was reporting both columns missing instead of one. The new error message is the correct one.