LevelDB: REPLACE doesn't work, produces ER_DUP_KEY

It might be a duplicate of MDEV-4036, but just in case it's not, I will file it separately. Besides, leveldb.test should probably contain both statements anyway.

Test case:

create table t1 (pk int primary key, i int) engine=LevelDB;

insert into t1 values (1,1);

replace into t1 values (1,100);

REPLACE fails with 1022: Can't write; duplicate key in table 't1'.

revision-id: psergey@askmonty.org-20130114131529-qv3531bw9fdklwob

revno: 4483

branch-nick: mysql-5.6-leveldb