Allow null as cache parameter (if no cache was found) and for cache removal.

lib/autoloader.php

@@ -150,7 +150,7 @@ class Autoloader {
 	 * @brief Sets the optional low-latency cache for class to path mapping.
 	 * @param \OC\Memcache\Cache $memoryCache Instance of memory cache.
 	 */
-	public function setMemoryCache(\OC\Memcache\Cache $memoryCache) {
+	public function setMemoryCache(\OC\Memcache\Cache $memoryCache = null) {
 		$this->memoryCache = $memoryCache;
 	}
 }