fix max_by_key mistake

src/stream/stream/max_by_key.rs

@@ -1,6 +1,6 @@
 use std::cmp::Ordering;
-use std::pin::Pin;
 use std::future::Future;
+use std::pin::Pin;
 
 use pin_project_lite::pin_project;
 
@@ -13,7 +13,7 @@ pin_project! {
     pub struct MaxByKeyFuture<S, T, K> {
         #[pin]
         stream: S,
-        max: Option<T>,
+        max: Option<(T, T)>,
         key_by: K,
     }
 }
@@ -37,24 +37,32 @@ where
     type Output = Option<S::Item>;
 
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        fn key<B, T>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
+            move |x| (f(&x), x)
+        }
+
         let this = self.project();
         let next = futures_core::ready!(this.stream.poll_next(cx));
 
         match next {
             Some(new) => {
-                let new = (this.key_by)(&new);
+                let (key, value) = key(this.key_by)(new);
                 cx.waker().wake_by_ref();
+
                 match this.max.take() {
-                    None => *this.max = Some(new),
+                    None => *this.max = Some((key, value)),
 
-                    Some(old) => match new.cmp(&old) {
-                        Ordering::Greater => *this.max = Some(new),
+                    Some(old) => match key.cmp(&old.0) {
+                        Ordering::Greater => *this.max = Some((key, value)),
                         _ => *this.max = Some(old),
                     },
                 }
                 Poll::Pending
             }
-            None => Poll::Ready(this.max.take()),
+            None => Poll::Ready(match this.max.take() {
+                None => None,
+                Some(max) => Some(max.1),
+            }),
         }
     }
 }