add `Response::into_boxed_body`

actix-http/CHANGES.md

@@ -5,6 +5,7 @@
 * Add timeout for canceling HTTP/2 server side connection handshake. Default to 5 seconds. [#2483]
 * HTTP/2 handshake timeout can be configured with `ServiceConfig::client_timeout`. [#2483]
 * Rename `body::BoxBody::{from_body => new}`. [#????]
+* `Response::into_boxed_body`. [#????]
 * `body::EitherBody` enum. [#????]
 
 ### Changed

src/response/response.rs

@@ -228,7 +228,14 @@ impl<B> HttpResponse<B> {
     }
 
     // TODO: into_body equivalent
-    // TODO: into_boxed_body
+
+    pub(crate) fn into_boxed_body(self) -> HttpResponse<BoxBody>
+    where
+        B: MessageBody + 'static,
+        B::Error: Into<Box<dyn StdError + 'static>>,
+    {
+        self.map_body(|_, body| BoxBody::new(body))
+    }
 
     /// Extract response body
     pub fn into_body(self) -> B {