Check that decorated connection responds to open_transactions before invoking

[rosa]

We have an instance of a Makara proxy we use for Redis, for read-write splitting. It looks like this:

class RedisProxy < Makara::Proxy

  # ...

  def connection_for(config)
    Redis.new(config)
  end

  # ...
end

The connection returned is an instance of a Redis client, which doesn't respond to open_transactions, so after updating to v0.5.1, any errors on Redis that would result in a Makara::Errors::BlacklistConnection, now result in a Redis::CommandError: ERR unknown command 'open_transactions'.

This PR addresses this. Let me know if you'd prefer an alternative approach or anything else!

[jorgemanrubia]

Any chance we can get this merged 🙏?

[jeremy]

Test case to demonstrate support and catch regression?

[mlarraz]

This looks good to me but as stated above an extra spec would be nice here.

[lewispb]

As discussed with @rosa, work continues in #341