Revert "Remove extra LEAF in memoize"

It was breaking for variable argument length methods
This reverts commit 85ebf7fffa.

src/utils/memoize.js

@@ -1,6 +1,16 @@
 
 import { Map } from 'immutable'
 
+/**
+ * The leaf node of a cache tree.
+ *
+ * An object, so that immutable maps will key it by reference.
+ *
+ * @type {Object}
+ */
+
+const LEAF = {}
+
 /**
  * An unique value used to detect cache misses
  *
@@ -26,7 +36,7 @@ function memoize(object, properties) {
     }
 
     object[property] = function (...args) {
-      const keys = [property, ...args]
+      const keys = [property, ...args, LEAF]
       this.__cache = this.__cache || new Map()
 
       const cachedValue = this.__cache.getIn(keys, NO_SET)