Table cell ID duplicated for whole month

I'm at work so can't roll a proper patch right now, but this is a trivial fix. In calendar_api.inc, around line 231 (as of the last 4.7.x release), there's this block of code:

$row[$x] = array(
  'class' => strtolower("$month_name ". $weekdays[$x]['day'] . ($curstamp == $today ? ' today' : '') . ($params['mini'] ? ' mini' : '')),
  'id' => strtolower($month_name . $day),
  'data' => $params['mini'] ? $content : '<div class="inner">'. $content .'</div>');

However, $day is null so all table cells get an ID of the 3 letter name of the month, which is invalid. The correct variable to use there is $cur_day.

There's not really a patch attached here, but it's a 4 character change so I don't feel guilty about it. :-)