Fix usage of hash function in _.memoize.

The function `_.memoize` has a bug where the first argument to the
function is used as the key when retrieving a result from the cache,
ignoring the hash function.

The fix is always to use the computed `address` as the key to the cache.

Confirmed the regression and fix with the updated unit test.

test/functions.js

@@ -127,6 +127,18 @@
     });
     hashed('yep');
     deepEqual(hashed.cache, {'YEP': 'yep'}, 'takes a hasher');
+
+    // Test that the hash function can be used to swizzle the key.
+    var objCacher = _.memoize(function(value, key) {
+      return {key: key, value: value};
+    }, function(value, key) {
+      return key;
+    });
+    var myObj = objCacher('a', 'alpha');
+    var myObjAlias = objCacher('b', 'alpha');
+    notStrictEqual(myObj, undefined, 'object is created if second argument used as key');
+    strictEqual(myObj, myObjAlias, 'object is cached if second argument used as key');
+    strictEqual(myObj.value, 'a', 'object is not modified if second argument used as key');
   });
 
   asyncTest('delay', 2, function() {

underscore.js

@@ -709,7 +709,7 @@
       var cache = memoize.cache;
       var address = hasher ? hasher.apply(this, arguments) : key;
       if (!_.has(cache, address)) cache[address] = func.apply(this, arguments);
-      return cache[key];
+      return cache[address];
     };
     memoize.cache = {};
     return memoize;