Swap the key check and use casefold in the test, as suggested in review

more_itertools/more.py

@@ -4279,7 +4279,7 @@ def all_unique(iterable, key=None):
     seenset_add = seenset.add
     seenlist = []
     seenlist_add = seenlist.append
-    for element in map(key, iterable) if key is not None else iterable:
+    for element in iterable if key is None else map(key, iterable):
         try:
             if element in seenset:
                 return False

tests/test_more.py

@@ -5160,7 +5160,7 @@ class AllUniqueTests(TestCase):
                 return False
 
             def __call__(self, x):
-                return x.lower()
+                return x.casefold()
 
         key = CaseFold()
         self.assertFalse(key)  # Sanity: the key itself is falsey