stables_mutation_test: more compare keys not representations
stables_mutation_test: more compare keys not representations

Commit 0fc4c369521f455f2858d1058f4fc3ead0f6e590 missed one place where
keys were compared using their byte representation. Fix that.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <1459778074-10759-4-git-send-email-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

raphaelsc/scylla,kjniemi/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,kjniemi/scylla,raphaelsc/scylla,scylladb/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,duarten/scylla