Allow multiple ToOneField/ToManyField on a single ModelResource to point to the same sub-Resource

If ModelResource_A has two or more ToOneField/ToManyField that point to the same ModelResource_B (e.g. field_foo = ToManyField(ModelResource_B), field_bar = ToManyField(ModelResource_B)) this fix allows additional uniqueness by updating the self.create_identifier() method to use the field_name property as part of the identification.  Without this, only the first field pointing to a particular resource is saved, and subsequent fields are ignored as their self.create_identifier() results were identical.