Set content-length for responses to HEAD requests

[davidpdrsn]

Hyper will automatically set the length if the body as an exact size, as determined via Body::size_hint, but since axum would use an empty body for HEAD requests the content-length would be set to 0 instead of the length of the original response body.

This fixes that by having axum set the content-length before removing the body.

I also changed RouteFuture to be the future we return regardless if we hit a route or the fallback. That way we only had to make this change in one place.

Required adding some B: HttpBody bounds in a few places so we can call size_hint but hyper already requires that so its unlikely to cause breakage.

Fixes #730

[davidpdrsn]

CI is unhappy since matchit 0.4.5 requires rust 2021. I'll get that resolved separately.