CLJS-1817: Strange result when assoc'ing 0 to persistent hash map

there was a bug in the port of Clojure's PersistentHashMap. When looking
up a node in a BitmapIndexedNode, we need to increment the found index
to compare the value. The relevant line in Clojure's codebase is:

https://github.com/clojure/clojure/blob/e547d/src/jvm/clojure/lang/PersistentHashMap.java#L940

Author: anmonteiro

src/main/cljs/cljs/core.cljs

@@ -6888,7 +6888,7 @@ reduces them without incurring seq initialization"
             (aset new-arr (inc len) val)
             (set! (.-val added-leaf?) true)
             (HashCollisionNode. nil collision-hash (inc cnt) new-arr))
-          (if (= (aget arr idx) val)
+          (if (= (aget arr (inc idx)) val)
             inode
             (HashCollisionNode. nil collision-hash cnt (clone-and-set arr (inc idx) val)))))
       (.inode-assoc (BitmapIndexedNode. nil (bitpos collision-hash shift) (array nil inode))

src/test/cljs/cljs/hash_map_test.cljs

@@ -29,3 +29,11 @@
          (-> (hash-map :a 1 :b 2 :c 3 :d 4 nil 5)
              (-iterator)
              (iter->set)))))
+
+(deftest test-cljs-1817
+  (let [cljscore-hash hash]
+    (with-redefs [hash (fn [x]
+                         (if (or (#{:a :b} x) 0)
+                           cljscore-hash))]
+      (let [x (hash-map :a :a :b -1)]
+        (is (= (assoc x :b :b) {:a :a :b :b}))))))