JQMIGRATE: jQuery.fn.offset() requires an element connected to a document

[MichaelRoosz]

This line in "src/position/viewport.js" throws the warning "JQMIGRATE: jQuery.fn.offset() requires an element connected to a document" when position.viewport is $('html'):

viewportOffset = viewport.offset() || adjusted;

This fixes it:

	var docElem = (viewport.ownerDocument || document).documentElement;
	if ($.contains(docElem, viewport)) {
            viewportOffset = viewport.offset() || adjusted;
	}
	else {
	    viewportOffset = adjusted;
	}