Because Redis uses a global key namespace, it's possible to
create a key of one type (e.g., a string) and then try to
work with is as if it were of another type (e.g., a hash).
I call this a "key type mismatch" situation.
Redis' handling of key type mismatches is a mess. Sometimes
it allows them (SET), sometimes it ignores them (MGET),
sometimes it forbids them (HSET).
Starting with this commit, Redka takes a more consistent
approach. Now you can use the same key for different data
types, and everything will work fine. With a small caveat:
the `type` column in the `rkey` table will store the last
modified type (it doesn't affect any operations though).
Having said that. Please don't use the same key for
different data types. It's a VERY bad idea.
I never quite understood the use case for MSETNX, so I searched
the Redis repo issues and StackOverflow. I didn't find a single
real-world scenario for using this command, so I decided to remove it.
