CheckNowCommandForm#init(): set submit label

... otherwise Form#handleRequest() bypasses Form#isSubmitted()
on API requests and considers the CheckNowCommandForm submitted.

refs #4281

modules/monitoring/application/forms/Command/Object/CheckNowCommandForm.php

@@ -19,6 +19,7 @@ class CheckNowCommandForm extends ObjectsCommandForm
     public function init()
     {
         $this->setAttrib('class', 'inline');
+        $this->setSubmitLabel($this->translate('Check now'));
     }
 
     /**