Fix plain GETEX wrongly removing expiration time

[enjoy-binbin]

The plain GETEX will remove the expiration time, since we
always do metadata.expire = expire and then write. In this
case, we will write expire = 0, causing the expiration time
to be erased.

Before:

127.0.0.1:6666> set foo bar ex 1000
OK
127.0.0.1:6666> ttl foo
(integer) 996
127.0.0.1:6666> getex foo
"bar"
127.0.0.1:6666> ttl foo
(integer) -1

After:

127.0.0.1:6666> set foo bar ex 1000
OK
127.0.0.1:6666> ttl foo
(integer) 998
127.0.0.1:6666> getex foo
"bar"
127.0.0.1:6666> ttl foo
(integer) 995