Implement FusedIterator for InternalMergeJoinBy

`I` and `J` are fused by `InternalMergeJoinBy` so we don't need them to implement `FusedIterator`.

src/merge_join.rs

@@ -1,5 +1,5 @@
 use std::cmp::Ordering;
-use std::iter::Fuse;
+use std::iter::{Fuse, FusedIterator};
 use std::fmt;
 use std::marker::PhantomData;
 
@@ -327,3 +327,10 @@ impl<I, J, F, T> Iterator for InternalMergeJoinBy<I, J, F>
         }
     }
 }
+
+impl<I, J, F, T> FusedIterator for InternalMergeJoinBy<I, J, F>
+where
+    I: Iterator,
+    J: Iterator,
+    F: OrderingOrBool<I::Item, J::Item, Out = T>,
+{}