Adds robustness to the zremrangebyrank method

[timherby]

I'm running into an error when removing the last item of a set, and then trying to trim a set down to a specific cap. With Redis, I don't need to check the length of a set before calling zremrangebyrank so I don't want to put that check in my production code. The set is missing because it gets removed by remove_key_for_empty_collection.

Here is the pseudocode for repro. This block will apply a diff update to an existing list, but fails in FakeRedis if the last item gets removed:

    if items_to_remove.any?
      list.redis.zrem list.key, items_to_remove.map { |i| list.to_redis(i) }
    end

    if items_to_add.any?
      list.merge items_to_add
    end

    # prune the list to fit within the cap
    list.remrangebyrank(0, -1 - Settings.buffer_size)

This pull request adds the same validation all the other methods use (see zrevrangebyscore), and also moves the method above next to its cousin zremrangebyscore rather than being next to the disssimilar zinterstore and zunionstore

[timherby]

Bump. Any chance we can get this pulled into master? Looks like it's useful to others as it was used by @mkdynamic.

[caius]

Many thanks for your contribution! 😀🎉