bug fix for _.memoize when key is derived from prototype

underscore.js

@@ -426,7 +426,7 @@
     hasher = hasher || _.identity;
     return function() {
       var key = hasher.apply(this, arguments);
-      return key in memo ? memo[key] : (memo[key] = func.apply(this, arguments));
+      return hasOwnProperty.call(memo, key) ? memo[key] : (memo[key] = func.apply(this, arguments));
     };
   };