Implement call method in pipelined block

[viralpraxis]

Resolves #278

Seems like everything is working; however, I'm not sure if moving #call from MockRedis to MockRedis::Database has some side-effects -- is there a reason call was separated from other commands?

I also changed send to public_send in call implementation (seems to be OK, I'm not sure if there are any edge cases when calling private is a usecase).

Also, I think we should allow mix-cased variants of Redis methods (RedisMock.new.call(["GET", 123]) does not work right now, but redis commands are case insensitive).

Usage example:

redis = MockRedis.new

redis.pipelined do |pipeline|
  pipeline.call(["get", "foo"])
  pipeline.call(["get", "bar"])
end

[sds]

Thanks!