Fix multi command

[sailor]

Actually, calling the Client#multi method fails as it tries to instantiate an inexistent Async::Redis::Context::Multi class.

 7.68s    error: Async::Task [oid=0x1c48] [pid=44325] [2020-02-13 22:48:03 +0100]
               |   NameError: uninitialized constant Async::Redis::Context::Multi
               |   → ./ruby-2.7.0@quiq/gems/async-redis-0.4.3/lib/async/redis/client.rb:94 in `multi'

It seems that this class is no more than the same thing as the Async::Redis::Context::Transaction class, that's what i aliased the multi method to the transaction one.

I'd like to know if it's the best approach.

Regards

[ioquatix]

This looks okay thanks.