Return value from `headerRenderer` in `MonthView` not used.

[chuckn0rris]

Try to specify any renderer, even empty method. See the line is disapeared. According to docs, we do not expect to have any value returned from that method.

Forum post

const calendar = new Calendar({
    appendTo : targetElement,
    height   : 560,
    date     : new Date(2020, 8, 1),
    modes    : {
        day    : null,
        week   : null,
        month  : {

            headerRenderer: (a,b,c) => {

            }
        },
        year   : null,
        agenda : null
    },
    events : [
        { startDate : '2020-09-02T07:00', duration : 5, durationUnit : 'h', name : 'Walk the dog', eventColor : 'yellow' },
        { startDate : '2020-09-10T09:00', duration : 2, durationUnit : 'h', name : 'Buy masks', eventColor : 'orange' },
        { startDate : '2020-09-14T07:00', duration : 1, durationUnit : 'h', name : 'Zoom meeting', eventColor : 'deep-orange' },
        { startDate : '2020-09-25T09:00', duration : 1, durationUnit : 'h', name : 'Get a haircut', eventColor : 'gray' }
    ]
});

[chuckn0rris]

Not a bug. Expected to return something from a headerRenderer to be shown. Nothing returned - nothing shown.

[ExtAnimal]

The docs are bad. We should make them true.

I suggest in CalendarPanel.js:

            if (me.headerRenderer) {
                cell.innerHTML = '';
                const header = me.callback(me.headerRenderer, me, [cell, columnIndex, cellDay]);
                if (header) {
                    if (typeof header === 'string') {
                        DomHelper.setInnerText(cell, header);
                    }
                    else {
                        DomHelper.createElement({
                            ...header,
                            parent : cell
                        });
                    }
            }
            else {
                DomHelper.setInnerText(cell, me.shortDayNames[cellDay]);
            }