Fix slice Rack middleware

[jodosha]

Fix

Ensure to properly mount Rack middleware in routing scope and slice.

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https://trello.com/c/4otUMpaN

[solnic]

I'm approving because the specs passed...otherwise I'm unable to review because I've no idea how things work "on the other side" 😅

[timriley]

Masterful work, @jodosha! I'm delighted with the end result here — this is such a powerful feature for Hanami!

From a quick glance at #1234, is it true that the other side of this work was mostly relocating code from hanami-api to hanami-router so that it could become accessible to us here in the hanami gem too?

Certainly having the Rack::Builder detail move outside of hanami and become a concern of Hanami::Middleware::App seems a win as well.

Anyway, very happy to see this go in 👍🏼

[jodosha]

@timriley Yes, we had the chance to extract code from hanami-api, which was the first gem to have this feature.