Fix a failure of a test case for a cluster client

[supercaracal]

The version 0.9.0 of redis-cluster-client was released. It made MSET, MGET and DEL commands able to pass multiple keys without a hashtag by using pipelining. Although the performance is degradated, I think it may be useful in some use cases. So, because the behavior was changed, This pull request fixes a failure of a test case for a cluster client.

[supercaracal]

  1) Failure:
TestClusterClientReplicas#test_client_can_command_with_replica [cluster/test/client_replicas_test.rb:19]:
--- expected
+++ actual
@@ -1,3 +1 @@
-# encoding: US-ASCII
-#    valid: true
-"1"
+nil

The above failure isn't related this pull request. It may be flaky. Please ignore it on this pull request. I'll look into and work on it if it occurs frequently.