Actually this was already the case for certain commands, but some of them
was left unguarded for such cases. This commit also fixes#109.
The behaviour of silently skipping key prefixing when a command has no
arguments may change in the future so we added explicit tests as guards
for future changes. Predis\Command\Processor\KeyPrefixProcessor will
continue to skip key prefixing on empty arguments, regardless.
When using UNSUBSCRIBE without a list of channels (which actually means
"unsubscribe from all the subscribed channels") Redis 2.6 does not
guarantee that channels are returned following the same order of
subscription.
Daniele Alessandri