Keys are not consistently converted to strings

[clemens]

Follow-up of #170.

The issue mentioned there also applies to other data types:

• sets
• sorted sets
• lists
• partially strings (e.g. incr, incrby, decr, decrby)
• presumably streams (haven't tested it, but the code looks like it)

Reproducible like so:

# assuming REDIS is an instance of MockRedis
irb(main):001:0> REDIS.sadd(:foo, [1, 2])
=> 2
irb(main):002:0> REDIS.zadd(:bar, [1, 2])
=> 1
irb(main):003:0> REDIS.keys
=> [:foo, :bar]
irb(main):004:0> REDIS.flushdb
=> "OK"
irb(main):005:0> REDIS.keys
=> [:foo, :bar]

It seems that pretty much everything except hashes (which were fixed in #173) and set are broken this way.

Proposed solution:

• extract a convert(key) utility method and wrap each usage of keys
• add a separate spec file that test each supported method with string and symbol keys OR add example to each single file

[sds]

Open to a pull request addressing this issue.