Fix redis-benchmark with AUTH

Previous benchmark AUTH implementation had a bug due to how redis-benchmark
generates queries.

The same problem applies to SELECT, and there was already a fix
in place for SELECT, so this just copies the SELECT methodology and
uses it for AUTH too.

Problem first reported at
https://github.com/antirez/redis/commit/8fcce90b259f85a4c9fb45301a1c1b67db8ebdb3#commitcomment-8606641

Output from `% ./redis-benchmark -c 1 -a bob -n 3 -t set -r 99999999` is now:

```haskell
1416415268.893890 [0 127.0.0.1:56169] "AUTH" "bob"
1416415268.893905 [0 127.0.0.1:56169] "SET" "key:000048417175" "xxx"
1416415268.894251 [0 127.0.0.1:56169] "SET" "key:000055642065" "xxx"
1416415268.894338 [0 127.0.0.1:56169] "SET" "key:000054091795" "xxx"
```

Prior to this commit, redis-benchmark would prepend AUTH to every command sent.