Generalize Reply impl for Box<dyn Reply>

We had a case where we were boxing up replies, and needed them to impl `Send + Sync` to use in other code. `Box<dyn Reply + Send + Sync>` unfortunately doesn't implement `Reply`, because the impl is too specific. This generalizes the impl of `Reply` to include any `Box<T>` where `T: Reply + ?Sized`.

src/reply.rs

@@ -289,7 +289,7 @@ pub trait Reply: BoxedReply + Send {
     */
 }
 
-impl Reply for Box<dyn Reply> {
+impl<T: Reply + ?Sized> Reply for Box<T> {
     fn into_response(self) -> Response {
         self.boxed_into_response(Internal)
     }