CRM-8252 Custom fields disappear from import mapping when disabled then re-enabled

Create a custom group, then create a custom field. Disable the field, then re-enable the field.

Import some data. Try to map data to that field. The field is not in the list.

Edit the custom field (no need to change anything), then save it.

Import some data again. The field has returned to the list.

Original discussion at http://forum.civicrm.org/index.php?topic=20205