CLJS-1209: Reduce produces additional final nil when used w/ eduction

Eduction IReduce implementations must wrap `f` in `completing`

Author: swannodette

src/main/cljs/cljs/core.cljs

@@ -8940,8 +8940,8 @@ reduces them without incurring seq initialization"
   (-seq [_] (seq (sequence xform coll)))
 
   IReduce
-  (-reduce [_ f] (transduce xform f coll))
-  (-reduce [_ f init] (transduce xform f init coll))
+  (-reduce [_ f] (transduce xform (completing f) coll))
+  (-reduce [_ f init] (transduce xform (completing f) init coll))
 
   IPrintWithWriter
   (-pr-writer [coll writer opts]

src/test/cljs/cljs/core_test.cljs

@@ -2814,6 +2814,14 @@
 (deftest test-cljs-1225
   (is (= (incme) 2)))
 
+(defn my-conj
+  [acc x]
+  (conj acc x))
+
+(deftest test-cljs-1209
+  (is (= (reduce my-conj [] (eduction (map identity) [1 2 3]))
+         [1 2 3])))
+
 (comment
   ;; ObjMap
   ;; (let [ks (map (partial str "foo") (range 500))