Can't download PDF from QuickBooks API: Accept header is overridden

[marcdejonge]

See: https://github.com/Lusitanian/PHPoAuthLib/blob/9735bf693d1d985e9173194d282df5d1e0d0c58a/src/OAuth/OAuth1/Service/QuickBooks.php#L117

Here the accept header is overridden, which means that we can never download the PDF file, since that needs the application/pdf accept header. I'll make a merge request to be able to override this.