fix: options object is modified by fullcalendar and prevent re-usage in ...

...a full refresh cycle for example.

So this sequence of code won't work as expected:

    $('#calendar').fullCalendar(options);
    $('#calendar').fullCalendar('destroy');
    $('#calendar').fullCalendar(options);

I expect it to redraw twice the same calendar.

But it wouldn’t, as ``options`` object is modified by ``fullcalendar`` (events gets removed, and maybe other things happens...).

A test file is provided. 

So this is my modest contribution if you would accept it. Any comments are welcome.