Replace deprecated Rack::Utils::HeaderHash with Rack::Headers

[basicfeatures]

Ref. #9

[pruzicka]

Can we merge this one please?

[marcotc]

Sorry for the long delay.

I'm working on getting Rack 3 support merged (including this fix).

I have to recreate CI because I was using Travis before, so it's not as trivial as I thought. I'll try to get it out in the next few days.

[cookrn]

@marcotc following up here, any chance you might be able to publish a new version including this fix? Thank you, I appreciate your work on this gem!

[marcotc]

Thank you for your work and patience, @basicfeatures, @pruzicka, and @cookrn! This should be now implemented in 2.0.0.