By convention all of the Redis command identifiers are uppercase.
Update examples.
Add the ability for the client to send raw commands to Redis.

When sending raw commands their arguments are not filtered, responses
are not parsed and key prefixes are not applied. The client also does
not throw any exception on Redis errors regardless of its settings.

The first parameter takes the raw arguments of the command (included
its identifier) as defined by the Redis documentation while the second
optional parameter is always populated by reference to indicate when
Redis actually returned an error response.

  $client->raw(['PING']);                // "PONG"
  $client->raw(['SET','foo','bar']);     // "OK"
  $client->raw(['GET','foo'], $err);     // "bar", $err=FALSE
  $client->raw(['LPUSH','foo',1], $err); // "WRONGTYPE...", $err=TRUE

Internally, this method creates instances of Predis\Command\RawCommand
that get passed to the underlying connection instance for execution as
if they were usual commands defined by Predis.

Raw commands work in both cluster and replication scenarios since they
are recognized by their command ID, but key prefixing is not supported
since it is done by the profile instance when instantiating commands.
Rework examples for plain Redis commands.
[tests] Normalize casing of command identifiers in constraint.

We do not care much about the casing of command IDs in our constraint,
so it makes no difference for use if it is "SET" or "set".
[tests] Rename test case classes.