Improve merge-with to work with map-like types that don't implement c…

…lojure.lang.IFn for lookup

Signed-off-by: Stuart Halloway <stu@thinkrelevance.com>

src/clj/clojure/core.clj

@@ -2273,7 +2273,7 @@
     (let [merge-entry (fn [m e]
 			(let [k (key e) v (val e)]
 			  (if (contains? m k)
-			    (assoc m k (f (m k) v))
+			    (assoc m k (f (get m k) v))
 			    (assoc m k v))))
           merge2 (fn [m1 m2]
 		   (reduce merge-entry (or m1 {}) (seq m2)))]