Only implement FusedIterator for TakeWhileInclusive when the inner iterator is fused

TakeWhileInclusive sets its `done` flag only when the predicate rejects an
element, not when the inner iterator returns None. With a non-fused inner
iterator it can therefore yield Some after returning None, which breaks the
FusedIterator contract it currently claims for every Iterator.

Bound the impl on `I: FusedIterator`, matching std::iter::TakeWhile.

Fixes #1088

Author: Patrick Wehbe

src/take_while_inclusive.rs

@@ -90,7 +90,7 @@ where
 
 impl<I, F> FusedIterator for TakeWhileInclusive<I, F>
 where
-    I: Iterator,
+    I: FusedIterator,
     F: FnMut(&I::Item) -> bool,
 {
 }

tests/quick.rs

@@ -1986,6 +1986,12 @@ quickcheck! {
         is_fused(a.fuse().interleave_shortest(b.fuse()))
     }
 
+    fn fused_take_while_inclusive(a: Iter<i16>) -> bool
+    {
+        !is_fused(a.clone().take_while_inclusive(|_| true)) &&
+        is_fused(a.fuse().take_while_inclusive(|_| true))
+    }
+
     fn fused_product(a: Iter<i16>, b: Iter<i16>) -> bool
     {
         is_fused(a.fuse().cartesian_product(b.fuse()))