Add support for an immutable Map to _.memoize.Cache.

lodash.src.js

@@ -8154,14 +8154,14 @@
       }
       var memoized = function() {
         var args = arguments,
-            cache = memoized.cache,
-            key = resolver ? resolver.apply(this, args) : args[0];
+            key = resolver ? resolver.apply(this, args) : args[0],
+            cache = memoized.cache;
 
         if (cache.has(key)) {
           return cache.get(key);
         }
         var result = func.apply(this, args);
-        cache.set(key, result);
+        memoized.cache = cache.set(key, result);
         return result;
       };
       memoized.cache = new memoize.Cache;