Fix core::iter::Fuse's Default impl to do what it's docs say it does.

Author: zachs18

library/core/src/iter/adapters/fuse.rs

@@ -199,7 +199,7 @@ impl<I: Default> Default for Fuse<I> {
     /// assert_eq!(iter.len(), 0);
     /// ```
     fn default() -> Self {
-        Fuse { iter: Default::default() }
+        Fuse { iter: Some(Default::default()) }
     }
 }