Also fix the opposite method.

src/TestSuite/IntegrationTestCase.php

@@ -561,7 +561,7 @@ abstract class IntegrationTestCase extends TestCase
         if (!$this->_response) {
             $this->fail('No response set, cannot assert content. ' . $message);
         }
-        $this->assertNotContains($content, $this->_response->body(), $message);
+        $this->assertNotContains($content, (string)$this->_response->body(), $message);
     }
 
     /**