Fix frozen literal warning on ruby 3.4

[Koilanetroc]

On ruby 3.4 I see this warning:

.asdf/installs/ruby/3.4.2/lib/ruby/gems/3.4.0/gems/em-http-request-1.1.7/lib/em-http/client.rb:222: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)

So this small fix should resolve the problem

[Koilanetroc]

and also maybe it's high time to add ruby 3.4 to CI matrix?

[cgunther]

I believe there's another occurence of the warning, if you could include that in your PR too:

em-http-request/lib/em-http/client.rb

Line 43 in ce363a7

@response = ''

Mutated here:

em-http-request/lib/em-http/client.rb

Line 222 in ce363a7

@response << data

[matheussilvasantos]

@igrigorik, could you have a look on this one?