fix keep-indexed transducer and add tests

Author: mpenet

pixie/stdlib.pxi

@@ -1762,7 +1762,7 @@ not enough elements were present."
   ([f]
    (fn [rf]
      (let [iv (atom -1)
-           rrf (preserving-reduced)]
+           rrf (preserving-reduced rf)]
        (fn
          ([] (rf))
          ([result] (rf result))

tests/pixie/tests/test-stdlib.pxi

@@ -22,7 +22,15 @@
 (t/deftest test-indexed
   (t/assert= (map-indexed (fn [& xs] xs) []) [])
   (t/assert= (map-indexed (fn [& xs] xs) [:a :b]) [[0 :a] [1 :b]])
-  (t/assert= (transduce (map-indexed (fn [& xs] xs)) conj [:a :b]) [[0 :a] [1 :b]]))
+  (t/assert= (transduce (map-indexed (fn [& xs] xs)) conj [:a :b]) [[0 :a] [1 :b]])
+
+  (t/assert= (keep-indexed (constantly true) []) [])
+  (t/assert= (keep-indexed (constantly nil) []) [])
+  (t/assert= (keep-indexed (fn [i x] [i x]) [:a :b]) [[0 :a] [1 :b]])
+
+  (t/assert= (transduce (keep-indexed (constantly true)) conj []) [])
+  (t/assert= (transduce (keep-indexed (constantly nil)) conj []) [])
+  (t/assert= (transduce (keep-indexed (fn [i x] [i x])) conj [:a :b]) [[0 :a] [1 :b]]))
 
 (t/deftest test-reductions
   (t/assert= (reductions + nil)