CLJS-1546: cljs.core/run! does not always return nil

The cljs.core/run! function only returns nil if the proc function
passed to it returns nil, but the docstring for run! indicates that it
returns nil.

Fixes this issue by explicitly returning nil. This also makes the
ClojureScript implementation of run! match that in Clojure 1.8.

Adds unit tests for run!, copying tests from Clojure that exhibit the
bug, while also adding a test for the primary functionality.

Author: mfikes

src/main/cljs/cljs/core.cljs

@@ -9293,8 +9293,8 @@ reduces them without incurring seq initialization"
   "Runs the supplied procedure (via reduce), for purposes of side
   effects, on successive items in the collection. Returns nil"
   [proc coll]
-  (reduce #(proc %2) nil coll))
-
+  (reduce #(proc %2) nil coll)
+  nil)
 
 (defprotocol IEncodeJS
   (-clj->js [x] "Recursively transforms clj values to JavaScript")

src/test/cljs/cljs/core_test.cljs

@@ -907,6 +907,16 @@
     (is (contains? "f" 0))
     (is (not (contains? "f" 55)))))
 
+(deftest test-run!
+  (testing "Testing run!"
+    (let [a (atom 0)]
+      (run! (fn [n]
+              (swap! a + n))
+        (range 5))
+      (is (= 10 @a)))
+    (is (nil? (run! identity [1])))
+    (is (nil? (run! reduced (range))))))
+
 (deftest test-distinct
   (testing "Testing distinct? & distinct"
     (is (distinct? 1 2 3))