Forward port zendframework/zendframework#3398

Author: Maks3w

src/Request.php

@@ -501,9 +501,7 @@ public function renderRequestLine()
     public function toString()
     {
         $str = $this->renderRequestLine() . "\r\n";
-        if ($this->headers) {
-            $str .= $this->headers->toString();
-        }
+        $str .= $this->getHeaders()->toString();
         $str .= "\r\n";
         $str .= $this->getContent();
         return $str;