fix: avoid setting content-length before middleware

[SabrinaJewson]

Motivation

Axum currently sets content-length automatically inside Route. However, if this occurs before middleware is run then should the middleware add a body to the request, Axum will avoid overwriting the content-length and so the user is stuck with an incorrect content-length, leading to panics in Hyper.

Solution

Only set content-length for top-level Routes.

(there are a couple miscellaneous changes as well; one is a one-line change in route.rs to avoid double-boxing the body for middleware, another removes the unused enum RouteFutureKind)

[jplatte]

I didn't dive into the details here, but nothing stands out as wrong and I'm pretty sure we have sufficient tests to cover this change (especially with you adding another one for the previous bug).

[jplatte]

Do you want to add a changelog entry though?

[SabrinaJewson]

Yup, good point – added.