CLJS-847: Safari toString fix

In Safari 6.0.X address problems with using toString by using string
concatenation instead.

src/clj/cljs/core.clj

@@ -1641,3 +1641,6 @@
   (lazy-cat xs ys zs) === (concat (lazy-seq xs) (lazy-seq ys) (lazy-seq zs))"
   [& colls]
   `(concat ~@(map #(core/list `lazy-seq %) colls)))
+
+(defmacro js-str [s]
+  (core/list 'js* "''+~{}" s))

src/cljs/cljs/core.cljs

@@ -2037,7 +2037,7 @@ reduces them without incurring seq initialization"
   ([] "")
   ([x] (if (nil? x)
          ""
-         (.toString x)))
+         (cljs.core/js-str x)))
   ([x & ys]
     (loop [sb (StringBuffer. (str x)) more ys]
       (if more