MDL-36351 Calendar: Poll interval for existing subscriptions will update

Pollinterval option value was not set, which was returning string, expected is int.
Added int as pollinterval option value.

calendar/renderer.php

@@ -800,6 +800,7 @@ protected function subscription_action_form($subscription, $courseid) {
                 if ($k == $subscription->pollinterval) {
                     $attributes['selected'] = 'selected';
                 }
+                $attributes['value'] = $k;
                 $html .= html_writer::tag('option', $v, $attributes);
             }
             $html .= html_writer::end_tag('select');