insert NULL on duplicate key update val bug.

[winkyao]

Please answer these questions before submitting your issue. Thanks!

drop table if exists test;
create table test(id int not null, val text, primary key(id));
insert into test values(1,'hello');
insert into test values(1, NULL) on duplicate key update val = VALUES(val);
select * from test; 
-- WRONG ANSWER: 1, ''
-- EXPECTED: 1, NULL 

select tidb_version();

Release Version: 1.0.0
Git Commit Hash: 1a175b562bc679694f606c336131a9373064b324
Git Branch: release-1.0
UTC Build Time: 2017-10-19 02:38:02