[IMP] purchase: method action_picking_create return picking_id

Returning `picking_id` can be useful for overrides.

In addition, when there is no return statement,
the method basically returns None.
As this is a public method (not beginning with '_'), it can
be called with an xmlrpc call, and `None` is not an
accepted return value for the xmlrpc protocol.

Closes #1714

addons/purchase/purchase.py

@@ -870,6 +870,7 @@ class purchase_order(osv.osv):
             }
             picking_id = self.pool.get('stock.picking').create(cr, uid, picking_vals, context=context)
             self._create_stock_moves(cr, uid, order, order.order_line, picking_id, context=context)
+        return picking_id
 
     def picking_done(self, cr, uid, ids, context=None):
         self.write(cr, uid, ids, {'shipped':1,'state':'approved'}, context=context)