Handle space after header

[andybalholm]

PDF Files produced by libtiff/tiff2pdf have an extra space after the file header. (In other words, the header is "%PDF-1.1 \n" instead of "%PDF-1.1\n". They work in most PDF viewers, but this package rejects them with the error "not a PDF file: invalid header". Would it be a good idea to relax the header check and allow the extra space?