Use std::future::ready (#231)

`std::future::ready` has been stable since 1.48 so since axum's MSRV is
1.51 we can use this rather the one from `futures_util`.

src/tests/handle_error.rs

@@ -1,5 +1,5 @@
 use super::*;
-use futures_util::future::{pending, ready};
+use std::future::{pending, ready};
 use tower::{timeout::TimeoutLayer, MakeService};
 
 async fn unit() {}