Merge pull request #9 from techhead/master

Clear underlying PSR-16 cache also

Author: byjg

src/Psr6/CachePool.php

@@ -166,6 +166,7 @@ public function hasItem($key)
      */
     public function clear()
     {
+        $this->_cacheEngine->clear();
         $this->bufferKeys = [];
         $this->buffer = [];
     }