fix utf16 in bookmarks

[SuddenGunter]

Fix for https://github.com/jung-kurt/gofpdf/issues/307

Tested on this code:

    pdf := gofpdf.New("P", "mm", "A5", "")
    pdf.AddUTF8Font("HAN NOM A", "", "hn.ttf")
    pdf.SetFont("HAN NOM A", "", 16)
    pdf.AddPage()
    pdf.Bookmark("1段落", 1, -1)
    pdf.Cell(0, 6, "2段落")
    pdf.Ln(50)
    pdf.AddPage()
    fileStr := example.Filename("Fpdf_Bookmark")
    err := pdf.OutputFileAndClose(fileStr)
    example.Summary(err, fileStr)
    // Output:
    // Successfully generated pdf/Fpdf_Bookmark.pdf

Result PDF:

I was unable to find ttf version of the font in original issue so used different one, but I think it won't be a problem.

[jung-kurt]

Many thanks, @SuddenGunter!