Enforce sprintf for exceptions

Author: dantleech

Response.php

@@ -355,7 +355,7 @@ public function send()
     public function setContent($content)
     {
         if (null !== $content && !is_string($content) && !is_numeric($content) && !is_callable(array($content, '__toString'))) {
-            throw new \UnexpectedValueException('The Response content must be a string or object implementing __toString(), "'.gettype($content).'" given.');
+            throw new \UnexpectedValueException(sprintf('The Response content must be a string or object implementing __toString(), "%s" given.', gettype($content)));
         }
 
         $this->content = (string) $content;