Custom fields that are set to be updated only by php hook are updated with default values when contact is edited in CRM Core

If a custom field with a multiple-choice set of values has a default value, that default value will be stored to the field if other fields on the contact are edited. Sounds OK, but probably shouldn't happen if the field is set not to be updated by CiviCRM – and in 3.01, the field will be updated even if set to be updated only by php hook. This update prevents extensions that link to CiviCRM from referencing the default value field in a useful way.

Very small bug.