Allow Contribution to process with amount of zero if price set contains items with cost of zero

A project I'm working on is using CiviContribute for the purchasing of a service, and makes use of a price set where some of the items are free. In order for this to work correctly, CRM_Contribute_BAO_Contribution_Utils would need to allow transactions with amount == 0 to create a contribution record. Since the contribution is a monetary contribution because some items have a cost, it seems to make sense to tie the processing condition to the presence of a price set item that has a cost of 0, rather than to let any transaction with a zero amount proceed.

Currently the monetary transaction with a zero amount causes the postProcess hook to be skipped entirely, which seems to necessitate the need for a change to the BAO.