CRM-19471 Custom relationships for custom contact types not available during export

This was found in version 4.6.9 and reproduced on the demo site.

To reproduce:
1) Create 2 Individual subtypes (patient and social worker)
2) Create custom relationship Social Worker for/Social Worker is with social worker as contact type a and patient as contact type b
3) Create Social Worker contact and Patient Contact
4) Create the Social Worker for/Social Worker is relationship between the contacts created in #3.
5) Advanced Search for Social Worker is relationship, select for export
6) Select fields for export
7) Select Social Worker as Record Type.

Only built-in relationships are available in the selector.

I fixed this for our client by changing CRM/Core/BAO/Mapping.php but I am not sure this is the best solution. I am also working with 4.6.x. In the buildMappingForm function (line 330 in version 4.6.9) I changed:
$relationshipTypes = CRM_Contact_BAO_Relationship::getContactRelationshipType(NULL, NULL, NULL, $value); to
$relationshipTypes = CRM_Contact_BAO_Relationship::getContactRelationshipType(NULL, NULL, NULL, $value, TRUE); which sets $all = TRUE in the function getContactRelationshipType.

This results in ALL relationships being added.

I can submit this change for 4.7.x if it is the best solution, but I doubt that it is. The client was able to get the data they needed, though.