Fix: different footer for first page

[ctrlaltca]

Sample 12 shows how to add a different header for the first page:

// Add first page header
$header = $section->addHeader();
$header->firstPage();
...
// Add header for all other pages
$subsequent = $section->addHeader();

This is supposed to work also for footers (since Header itself is a subclass of Footer), but it's currently broken because Section::hasDifferentFirstPage() only checks if a first-page specific header exists, but doesn't check for a first-page specific footer, too.

This PR adds the missing check, fixing the rendering of footers with type Footer::FIRST.

[ctrlaltca]

As a workaround until this gets fixed, creating an empty header for the first page is enough to get the footers rendered correctly.