Actually I was referring only to `prefixKeys()` but well there's also `filterArguments()` to be overridden even if for a completely different reason: allowing to pass arguments to the command doing both `$client->pfcount($key1, $key2)` and `$client->pfcount(array($key1, $key2))`. As for `PFADD` and `PFMERGE`, their `filterArguments()` method should be overridden to use `self::normalizeVariadic()`.

I'd implement the changes to `filterArguments()` in a separate commit.