separate merge arities

Author: swannodette

src/main/cljs/cljs/core.cljs

@@ -9292,9 +9292,11 @@ reduces them without incurring seq initialization"
   "Returns a map that consists of the rest of the maps conj-ed onto
   the first.  If a key occurs in more than one map, the mapping from
   the latter (left-to-right) will be the mapping in the result."
-  [& maps]
-  (when (some identity maps)
-    (reduce #(conj (or %1 {}) %2) maps)))
+  ([] nil)
+  ([x] x)
+  ([x y] (if (and x y) (into x y) (or x y)))
+  ([x y & maps]
+   (reduce merge (merge x y) maps)))
 
 (defn merge-with
   "Returns a map that consists of the rest of the maps conj-ed onto