Add support for ('setDate', null)

When ('setDate', null) is called it deletes only date part of alternate field but time still remains.
This patch should fix the problem and properly delete whole alternate field in case that null is set as new date.

Author: jhron

src/jquery-ui-timepicker-addon.js

@@ -1494,7 +1494,7 @@
 			}
 		}
 		else {
-			$.datepicker._base_updateAlternate(inst);
+			$(altField).val( inst.input.val() ? altFormattedDateTime : "");
 		}
 	};