Don't use column name for pk

Gets the attribute name of the model's pk column, rather than the
column's name (which can differ).

This probably needs to happen for inherited keys, too, but I have not
tested.

flask_admin/contrib/sqla/tools.py

@@ -31,7 +31,7 @@ def get_primary_key(model):
                 if is_inherited_primary_key(p):
                     pks.append(get_column_for_current_model(p).key)
                 else:
-                    pks.append(p.columns[0].key)
+                    pks.append(p.key)
     if len(pks) == 1:
         return pks[0]
     elif len(pks) > 1: