Add extra test case for cache registry setter and update docblock.

src/Cache/Cache.php

@@ -118,7 +118,7 @@ class Cache
      * Also allows for injecting of a new registry instance.
      *
      * @param \Cake\Core\ObjectRegistry $registry Injectable registry object.
-     * @return \Cake\Cache\CacheRegistry
+     * @return \Cake\Core\ObjectRegistry
      */
     public static function registry(ObjectRegistry $registry = null)
     {

tests/TestCase/Cache/CacheTest.php

@@ -15,6 +15,7 @@
 namespace Cake\Test\TestCase\Cache;
 
 use Cake\Cache\Cache;
+use Cake\Cache\CacheRegistry;
 use Cake\Cache\Engine\FileEngine;
 use Cake\Core\App;
 use Cake\Core\Configure;
@@ -569,4 +570,17 @@ class CacheTest extends TestCase
     {
         $this->assertInstanceOf('Cake\Cache\CacheRegistry', Cache::registry());
     }
+
+    /**
+     * test registry method setting
+     *
+     * @return void
+     */
+    public function testRegistrySet()
+    {
+        $registry = new CacheRegistry();
+        Cache::registry($registry);
+
+        $this->assertSame($registry, Cache::registry());
+    }
 }