Bugfix sebastianbergmann#2705: if 'stderr=false' is set in phpunit.xm…

…l, it is ignored and considered true.

src/TextUI/TestRunner.php

@@ -279,7 +279,7 @@ public function doRun(Test $suite, array $arguments = [], $exit = true)
                 }
 
                 $this->printer = new $printerClass(
-                    isset($arguments['stderr']) ? 'php://stderr' : null,
+                    (isset($arguments['stderr']) && $arguments['stderr'] === true) ? 'php://stderr' : null,
                     $arguments['verbose'],
                     $arguments['colors'],
                     $arguments['debug'],