CIVICRM-230 Fix: Import Contributions using External Id's causes soft credit to be wrongly associated

When importing contributions where BOTH the donor and the soft credit recipient are specified by External ID, then the contribution is allocated to the soft credit user not the donor.

Steps to reproduce:

1. Choose two users and set external id's of "ext-1", "ext-2"
2. Create import.csv as:
   "Total Amount","Financial Type","External Identifier","Soft Ext Id"
   10,"Donation","ext-1","ext-2"
3. Go to Import Contributions. For the field mapping for "Soft Ext Id", select "Soft Credit", "External id"

The correct result would be a contribution created for ext-1 with a soft credit to ext-2
What actually happens is a contribution for ext-2 with a soft credit to the same contact

This only seems to happen when both are identified by external id's. Matching by email or contact id works as expected.

Tested on d46.demo.civicrm.org - see http://d46.demo.civicrm.org/civicrm/contact/view?reset=1&cid=158 until it resets ...