Maintenance: Fix warnings that are shown when using >= Ruby 3.4.

[fliebe92]

 /Users/xxx/.rbenv/versions/3.4.5/lib/ruby/gems/3.4.0/gems/em-websocket-0.5.3/lib/em-websocket/masking04.rb:29: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)

 /Users/xxx/.rbenv/versions/3.4.5/lib/ruby/gems/3.4.0/gems/em-websocket-0.5.3/lib/em-websocket/framing07.rb:135: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)

This PR addresses both warnings.