fix printing ObjectWrapper errors in PHP 7

src/Storm/Test/ObjectWrapper.php

@@ -54,7 +54,7 @@ class Storm_Test_Mock extends StdClass {
   protected function _argToSring($arg) {
     return is_object($arg)
       ? get_class($arg)
-      : $arg;
+      : print_r($arg, true);
   }
 }