MDL-61408 qbehaviour: Correct "Check" button styling.

question/behaviour/rendererbase.php

@@ -234,7 +234,7 @@ protected function submit_button(question_attempt $qa, question_display_options
             'id' => $qa->get_behaviour_field_name('submit'),
             'name' => $qa->get_behaviour_field_name('submit'),
             'value' => get_string('check', 'question'),
-            'class' => 'submit btn',
+            'class' => 'submit btn btn-default',
         );
         if ($options->readonly) {
             $attributes['disabled'] = 'disabled';