Clear cached amount when resetting to draft sale, purchase and purchase requisition

Closes #13296
(grafted from 8e441329d11b93ceafa47d553cb23883e5b4ec2f)

--HG--
branch : 7.0

modules/purchase/purchase.py

@@ -867,7 +867,11 @@ class Purchase(
     @Workflow.transition('draft')
     @reset_employee('quoted_by', 'confirmed_by')
     def draft(cls, purchases):
-        pass
+        cls.write(purchases, {
+                'tax_amount_cache': None,
+                'untaxed_amount_cache': None,
+                'total_amount_cache': None,
+                })
 
     @classmethod
     @ModelView.button

modules/purchase_requisition/purchase.py

@@ -344,7 +344,11 @@ class PurchaseRequisition(Workflow, ModelSQL, ModelView):
     @Workflow.transition('draft')
     @reset_employee('approved_by', 'rejected_by')
     def draft(cls, requisitions):
-        pass
+        cls.write(requisitions, {
+                'tax_amount_cache': None,
+                'untaxed_amount_cache': None,
+                'total_amount_cache': None,
+                })
 
     @classmethod
     @ModelView.button

modules/sale/sale.py

@@ -1001,7 +1001,11 @@ class Sale(
     @Workflow.transition('draft')
     @reset_employee('quoted_by', 'confirmed_by')
     def draft(cls, sales):
-        pass
+        cls.write(sales, {
+                'tax_amount_cache': None,
+                'untaxed_amount_cache': None,
+                'total_amount_cache': None,
+                })
 
     @classmethod
     @ModelView.button