Adjust default lead time to be None

It's potentially confusing to have it default to a value as you might
miss this in your project.

oscar/apps/partner/wrappers.py

         return _("Not available")
 
     def dispatch_date(self, stockrecord):
+        """
+        We don't provide a default value as it could be confusing.  Subclass
+        and override this method to provide estimated dispatch dates
+        """
         return None
 
     def lead_time(self, stockrecord):
-        return 1
+        """
+        We don't provide a default value as it could be confusing.  Subclass
+        and override this method to provide estimated dispatch dates
+        """
+        return None
 
     def calculate_tax(self, stockrecord):
         return D('0.00')

tests/unit/partner/wrapper_tests.py

     def test_returns_no_estimated_dispatch_date(self):
         self.assertIsNone(self.wrapper.dispatch_date(self.record))
 
+    def test_returns_no_estimated_lead_time(self):
+        self.assertIsNone(self.wrapper.lead_time(self.record))
+
     def test_returns_zero_tax(self):
         self.assertEqual(D('0.00'), self.wrapper.calculate_tax(self.record))
 
     def test_returns_no_estimated_dispatch_date(self):
         self.assertIsNone(self.wrapper.dispatch_date(self.record))
 
+    def test_returns_no_estimated_lead_time(self):
+        self.assertIsNone(self.wrapper.lead_time(self.record))
+
     def test_returns_zero_tax(self):
         self.assertEqual(D('0.00'), self.wrapper.calculate_tax(self.record))
 
     def test_returns_no_estimated_dispatch_date(self):
         self.assertIsNone(self.wrapper.dispatch_date(self.record))
 
+    def test_returns_no_estimated_lead_time(self):
+        self.assertIsNone(self.wrapper.lead_time(self.record))
+
     def test_returns_zero_tax(self):
         self.assertEqual(D('0.00'), self.wrapper.calculate_tax(self.record))
 
     def test_returns_no_estimated_dispatch_date(self):
         self.assertIsNone(self.wrapper.dispatch_date(self.record))
 
+    def test_returns_no_estimated_lead_time(self):
+        self.assertIsNone(self.wrapper.lead_time(self.record))
+
     def test_returns_zero_tax(self):
         self.assertEqual(D('0.00'), self.wrapper.calculate_tax(self.record))