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/resource.rs

@@ -54,7 +54,10 @@ pub struct Resource<B = Body> {
     pub(crate) router: Router<B>,
 }
 
-impl<B: Send + 'static> Resource<B> {
+impl<B> Resource<B>
+where
+    B: axum::body::HttpBody + Send + 'static,
+{
     /// Create a `Resource` with the given name.
     ///
     /// All routes will be nested at `/{resource_name}`.