Fix kwargs delegation for redis 4.2.0 with Ruby 2.8.0-dev

[kamipo]

redis 4.2.0 with Ruby 2.8.0-dev has broken Rails CI.

redis/redis-rb@1e5d0a1
https://buildkite.com/rails/rails/builds/69984#fc71e8e4-bf32-425b-a466-6487d8fa001d/1793-1824

This allows kwargs delegation for redis 4.2.0 client.

I have found #169 after this change has finished.

Because #169 and this have different directions,

• Test against ruby 2.7 and fix warnings #169: force add kwargs flag (i.e. end users doesn't need to care about the command should be passed kwargs or not),
• this change here: respect passed arguments (i.e. end users need to care about the command should be passed kwargs or not),

so I'll leave this as another choice.

[rafaelfranca]

I went with #169. Thank you for the PR.