Ensure getHeaders() always returns an instance of Headers

src/Storage/Part.php

@@ -266,6 +266,7 @@ public function countParts()
      * Lazy-loads if not already attached.
      *
      * @return Headers
+     * @throws Exception\RuntimeException
      */
     public function getHeaders()
     {
@@ -277,6 +278,11 @@ public function getHeaders()
                 $this->headers = new Headers();
             }
         }
+        if (!($this->headers instanceof Headers)) {
+            throw new Exception\RuntimeException(
+                '$this->headers must be an instance of Headers'
+            );
+        }
 
         return $this->headers;
     }

test/Storage/MessageTest.php

@@ -276,6 +276,15 @@ public function testEmptyHeader()
         $this->setExpectedException('Zend\\Mail\\Exception\\InvalidArgumentException');
         $message->subject;
     }
+
+    public function testWrongHeaderType()
+    {
+        $this->setExpectedException('Zend\\Mail\\Exception\\RuntimeException');
+        $badMessge = unserialize(
+            "O:25:\"Zend\Mail\Storage\Message\":9:{s:8:\"\x00*\x00flags\";a:0:{}s:10:\"\x00*\x00headers\";s:16:\"Yellow submarine\";s:10:\"\x00*\x00content\";N;s:11:\"\x00*\x00topLines\";s:0:\"\";s:8:\"\x00*\x00parts\";a:0:{}s:13:\"\x00*\x00countParts\";N;s:15:\"\x00*\x00iterationPos\";i:1;s:7:\"\x00*\x00mail\";N;s:13:\"\x00*\x00messageNum\";i:0;}"
+        );
+        $this->getHeaders();
+    }
 
     public function testEmptyBody()
     {