BaseIPN fails when receipt_date is set

I've been getting inconsistent results on this - also in the test suite. But, setting receipt_date causes the test to fail if run in isolation but not with other tests. It's something to do with the DAO retrieving dates in a different format than it required it in - we also hit it here

https://github.com/civicrm/civicrm-core/commit/d5d148ab08ad49dab36ed055a963bb784e006909#diff-ccd0e4b1faa09c1f991a7d87b135e6b5

However, there is an existing hacky method in the BaseIPN class that I can extend to this field