[FIX] mrp_repair: group multiple repairs into one invoice

While `action_repair_invoice_create` is called on active_ids, it was working
only on one record at a time

Closes #18158

addons/mrp_repair/models/mrp_repair.py

@@ -226,11 +226,12 @@ class Repair(models.Model):
 
     @api.multi
     def action_repair_invoice_create(self):
-        self.action_invoice_create()
-        if self.invoice_method == 'b4repair':
-            self.action_repair_ready()
-        elif self.invoice_method == 'after_repair':
-            self.write({'state': 'done'})
+        for repair in self:
+            repair.action_invoice_create()
+            if repair.invoice_method == 'b4repair':
+                repair.action_repair_ready()
+            elif repair.invoice_method == 'after_repair':
+                repair.write({'state': 'done'})
         return True
 
     @api.multi