Fetch single vendorItems record by sku instead of upc

b/c that is what we must use as PK when updating the record etc.

corepos/api.py

@@ -468,13 +468,13 @@ class CoreWebAPI(object):
         result = self.parse_response(response)
         return [json.loads(rec) for rec in result]
 
-    def get_vendor_item(self, upc, vendorID, **columns):
+    def get_vendor_item(self, sku, vendorID, **columns):
         """
         Fetch an existing VendorItem record from CORE.
 
         :returns: Either a vendor item dict record, or ``None``.
         """
-        columns['upc'] = upc
+        columns['sku'] = sku
         columns['vendorID'] = vendorID
         params = {
             'entity': 'VendorItems',