Fix for PDI-3813 when external id reference fields are null values.  Bug states that when reference fields have null values the system throw an exception because the fieldtonull element would contain the wrong field name:
For example, before this fix - this would be submitted to Salesforce API when the Account field was null:
<ns7:fieldsToNull xmlns:ns7="urn:sobject.partner.soap.sforce.com">Account:External_Id__c:Account</ns7:fieldsToNull>
Should have been:
<ns7:fieldsToNull xmlns:ns7="urn:sobject.partner.soap.sforce.com">Account</ns7:fieldsToNull>

For custom relationships WAS:
<ns7:fieldsToNull xmlns:ns7="urn:sobject.partner.soap.sforce.com">Account:External_Id__c:Custom_Account__r</ns7:fieldsToNull>
Should have been:
<ns7:fieldsToNull xmlns:ns7="urn:sobject.partner.soap.sforce.com">Custom_Account__c</ns7:fieldsToNull>

Note that the custom relationship field must end in __r when assign the value but must end is __c when setting it to null.