[#32642] Warning if string is passed as $attrib

Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_id=8103&tracker_item_id=32642

replace: https://github.com/joomla/joomla-cms/pull/2463

## How to test

### 1. way
1) BE --> Content --> Article Manager --> Add New Article --> Tab: Publishing
2) make sure the calendar works ok.
3) add into this file at this place: https://github.com/joomla/joomla-cms/blob/staging/libraries/cms/html/html.php#L959

$attribs = 'example';

4) refresh the page
5) make sure that the calendar is broken
6) apply patch
7) redo 3)
8) refresh the page
9) make sure that all works as bevor

### Alternative way:
Code: <?php echo JHtml::calendar(null, 'test', 'id-test', '%Y-%m-%d', 'class="blah" readonly="true"'); ?>

Insert this code into a view or index.php of your template (i.e. some php file that is viewable). Note the 3 php errors. Apply the PR. Errors go away.