Passing `info a b c` is the same as passing plain `info`,
since we only checked that args_.size() == 2, by default it
will output all.

In this PR, we will reject args_.size() > 2 and return a
syntax error. This is consistent with Redis 6.2 behavior,
however, it is inconsistent with Redis 7.0, Redis 7.0 supports
passing `info server clients ...`.

Because we don't plan to do such support for the time being,
we will first fix the parts that are obviously problematic.