Fix cookie() fail when first time set session

https://github.com/walkor/webman-framework/issues/22

Protocols/Http/Response.php

@@ -189,7 +189,7 @@ class Response
      * @return $this
      */
     public function withHeaders($headers) {
-        $this->_header = \array_merge($this->_header, $headers);
+        $this->_header = \array_merge_recursive($this->_header, $headers);
         return $this;
     }