Exporting related contacts' prefix exports prefix_id

I suspect this is part of CRM-14259, but a less common use case so it didn't get caught.

To replicate:

• Identify a contact who has a relationship with someone with an individual prefix. E.g. John Doe is the Parent of Ms. Jane Doe.
• Export the first contact (John Doe); export related "Parent Of", including the Individual Prefix.

Instead of exporting the prefix ("Ms."), it exports the prefix_id value instead. I imagine other pseudoconstants work similarly.