Make sure count mode is preserved when making new inventory batch

i.e. even if only one count mode is allowed for the user

tailbone/views/inventory.py

@@ -225,7 +225,8 @@ class InventoryBatchView(BatchMasterView):
         f.set_enum('mode', modes)
         f.set_label('mode', "Count Mode")
         if len(modes) == 1:
-            f.set_readonly('mode')
+            f.set_widget('mode', forms.widgets.ReadonlyWidget())
+            f.set_default('mode', list(modes)[0])
 
         # total_cost
         if self.creating: