Update fflib_SObjectUnitOfWork.cls

Changes to allow self-referencing objects to be inserted correctly in a single UoW. e.g.

fflib_SObjectUnitOfWork uow = 
	new fflib_SObjectUnitOfWork(new Schema.SObjectType[] 
		{ Account.SObjectType });

Account parent = new Account(Name = 'Test Parent');
uow.registerNew(parent);
uow.registerNew(new Account(Name = 'Test Child'), Account.ParentId, parent);
uow.registerNew(new Account(Name = 'Test Child'), Account.ParentId, parent);
uow.commitWork();