Add support for optional count arg in rpop

[EmilioCristalli]

rpop accepts an optional count argument to indicate how many elements should be removed and returned from the list

See https://github.com/redis/redis-rb/blob/9938411bd44383b795e05df900abce4df66daaef/lib/redis/commands/lists.rb#L114

Also had to change the shared examples a little bit to be able to pass the arguments they use and make a more accurate expectation on the error.
I think the args_for_method is making an assumption when the arity < 0 and always using [1, 2] (+ the key), but that doesn't work in all cases. In particular, rpop now has arity -2 (because it has 1 required arg + 1 optional) so calling rpop(key, 1, 2) was causing an argument error instead of Redis::CommandError (which we expect because of the redis value not being a list).

At first I tried to change args_for_method but it made other tests fail. And i suspect it won't be possible to have a generic args generator only based on arity (because some methods for example accept *args but the logic requires 1 or 2 args)

That's why i thought it might be a good idea for each test that includes the shared example to indicate what the correct args to make a valid call should be, but let me know what you think!

[sds]

Thanks!