Readonly parameter ignored by datepicker

If a input tag has the readonly parameter set to true, datepicker still works and changes its value. This differs from disabled, because readonly still posts the form data but disabled does not.

How to reproduce: <input class="datepicker" readonly="readonly" type="text"/>

This seems to be somewhat related to #5644. Proposed fix: alter _connectDatepicker() and append at its end the following code:

if( input.attr('readonly')) {
    this._disableDatepicker( target );
}

and change ._disableDatepicker so that it does not call these lines if readonly is enabled:

target.disabled = true;
inst.trigger.filter('button').
each(function() { this.disabled = true; }).end().
filter('img').css({opacity: '0.5', cursor: 'default'});