Fix RKReplaceAssignmentPolicy.  The problem was that new values for the relationship were gathered before the old values were deleted.  Therefore if one of the new values was the same as the old value, the valueForRelationship collection would contain a deleted managed - not good.

Also fixed:
* The if statement near line 674 was incorrectly negated
 * Setting the relationship value around line 676 was redundant since it was done in the mapCoreDataToManyRelationshipValue:withMapping method.