showButtonPanel attribute in p:calendar: styling of buttons

[wincornixdorf]

The buttons for "Current Date" and "Close" that appear when showButtonPanel is set to true are unstyled.
It seems that the CSS class "ui-button" is missing.

Steps to reproduce:
Simple change the default forms.xhtml from
<p:calendar id="popup" />
to
<p:calendar id="popup" showButtonPanel="true" />
and open the calendar.

[mert.sincan]

Please see;
viewtopic.php?f=42&t=50645&p=155430