Fix intermittent loan transfer spec failure

When a loan is transferred the last_modified_at column is not copied across to the new loan, but we weren't whitelisting it as a not-copied field in the spec.

This was passing intermittently as a loan transfer causes a `LoanStateChange.log` call for both loans. This updates the last_modified_at on both loans and they often end up with the same value. I've blacklisted the column in the spec as we don't actually care if the two values are equal (though we would expect them to be close).