[fix] [minor] exchange_rate should not be no_copy field

buying/doctype/purchase_order/purchase_order.txt

@@ -2,7 +2,7 @@
  {
   "creation": "2013-05-21 16:16:39", 
   "docstatus": 0, 
-  "modified": "2013-07-09 12:50:00", 
+  "modified": "2013-07-19 16:14:49", 
   "modified_by": "Administrator", 
   "owner": "Administrator"
  }, 
@@ -198,7 +198,7 @@
   "fieldtype": "Float", 
   "hidden": 0, 
   "label": "Exchange Rate", 
-  "no_copy": 1, 
+  "no_copy": 0, 
   "oldfieldname": "conversion_rate", 
   "oldfieldtype": "Currency", 
   "print_hide": 1,