Pdf class keeps state between two consecutive calls to generate method

When calling `Pdf::generate(...)` with `footer-html` or `header-html` options, the class remembers if they were set, to remove the temporary files afterwise.

If you call `Pdf::generate(...)` a second time, without `footer-html` or `header-html` options, it's still trying to remove some generated files, although they are not there. This results in PHP warnings and notices.

It should be possible to generate PDFs with different options, I guess...

I'll try to provide some tests for this issue.