Fix ruby warnings

[palkan]

This is an upgraded version of #150.

The only warnings left are from third-party gems:

$ RUBYOPT="-W" be rspec

/opt/homebrew/lib/ruby/gems/3.3.0/gems/nkeys-0.1.0/lib/nkeys/keypair.rb:56: warning: method redefined; discarding old public_key
/opt/homebrew/lib/ruby/gems/3.3.0/gems/nkeys-0.1.0/lib/nkeys/keypair.rb:78: warning: method redefined; discarding old private_key

/Users/vladimirdementyev/dev/nats-pure.rb/spec/support/dns.rb:28: warning: method redefined; discarding old initialize
/opt/homebrew/lib/ruby/gems/3.3.0/gems/resolv-replace-0.1.1/lib/resolv-replace.rb:23: warning: previous definition of initialize was here

...

Closes #150

[wallyqs]

LGTM