Set content-length for responses to HEAD requests (#734)

* Set `content-length` internally in `Router`

* changelog

* require `B: HttpBody` at the `impl`

* add changelog refs

axum-extra/src/routing/mod.rs

@@ -36,7 +36,7 @@ pub trait RouterExt<B>: sealed::Sealed {
 
 impl<B> RouterExt<B> for Router<B>
 where
-    B: Send + 'static,
+    B: axum::body::HttpBody + Send + 'static,
 {
     fn with<T>(self, routes: T) -> Self
     where