mibioa/query: fix regression QuerySet.iterate() crash in certain cases

Commit 6eaf293 caused crash in e.g. values_list().iterate(cache=True)
as there a FK field's attname is automatically listed as output field.
This fix adds conditional code allowing either field_name or attname, as
needed.

mibios/query.py

@@ -351,9 +351,14 @@ def update_bin(self, queryset):
             get_fk = attrgetter(self.attname)
         else:
             # values list queryset
-            get_fk = itemgetter(
-                queryset.get_output_field_names().index(self.field_name)
-            )
+            try:
+                pos = queryset.get_output_field_names().index(self.field_name)
+            except ValueError:
+                # field_name is not in list, try attname
+                pos = queryset.get_output_field_names().index(self.attname)
+
+            get_fk = itemgetter(pos)
+            del pos
 
         missing = set()
         for i in queryset:
@@ -508,7 +513,7 @@ def _iter(self):
         """
         The iterator/generator yielding the data.
 
-        inheriting classes must implement this
+        Must be provided by implementing class.
         """
         raise NotImplementedError