add responder for unit

CHANGES.md

@@ -3,6 +3,7 @@
 ## Unreleased - 2021-xx-xx
 ### Added
 * Option to allow `Json` extractor to work without a `Content-Type` header present. [#2362]
+* Implemented responder for `()` to respond with `204 No Content`
 
 ### Changed
 * Associated type `FromRequest::Config` was removed. [#2233]

src/responder.rs

@@ -116,6 +116,12 @@ impl<T: Responder> Responder for (T, StatusCode) {
     }
 }
 
+impl Responder for () {
+    fn respond_to(self, req: &HttpRequest) -> HttpResponse {
+        HttpResponse::new(StatusCode::NO_CONTENT)
+    }
+}
+
 macro_rules! impl_responder {
     ($res: ty, $ct: path) => {
         impl Responder for $res {