various fixes to MessageBody::complete_body (#2519)

actix-http/src/body/boxed.rs

@@ -57,27 +57,7 @@ impl MessageBody for BoxBody {
     }
 
     fn take_complete_body(&mut self) -> Bytes {
-        debug_assert!(
-            self.is_complete_body(),
-            "boxed type does not allow taking complete body; caller should make sure to \
-            call `is_complete_body` first",
-        );
-
-        // we do not have DerefMut access to call take_complete_body directly but since
-        // is_complete_body is true we should expect the entire bytes chunk in one poll_next
-
-        let waker = futures_util::task::noop_waker();
-        let mut cx = Context::from_waker(&waker);
-
-        match self.as_pin_mut().poll_next(&mut cx) {
-            Poll::Ready(Some(Ok(data))) => data,
-            _ => {
-                panic!(
-                    "boxed type indicated it allows taking complete body but failed to \
-                    return Bytes when polled",
-                );
-            }
-        }
+        self.0.take_complete_body()
     }
 }