Encrypt hex literals when needed

[leppie]

Hex literal strings werent getting encrypted so it would break images with indexed colorspace.

Sample program:

using System;
using PdfSharp.Pdf;
using PdfSharp.Pdf.IO;

class Program
{
    static void Main()
    {
        var ts = (DateTime.Now - new DateTime(1970, 1, 1)).TotalMilliseconds;

        using (var targetDoc = new PdfDocument())
        {
            using (var src = PdfReader.Open("test.pdf", PdfDocumentOpenMode.Import))
            {
                for (var i = 0; i < src.PageCount; i++)
                {
                    targetDoc.AddPage(src.Pages[i]);
                }
            }

            targetDoc.Save($"out1-{ts:F0}.pdf");

            targetDoc.SecuritySettings.UserPassword = "1234";

            targetDoc.Save($"out2-{ts:F0}.pdf");
        }

        // round trip test
        using (var targetDoc = new PdfDocument())
        {
            using (var src = PdfReader.Open($"out2-{ts:F0}.pdf", "1234", PdfDocumentOpenMode.Import))
            {
                for (var i = 0; i < src.PageCount; i++)
                {
                    targetDoc.AddPage(src.Pages[i]);
                }
            }

            targetDoc.Save($"out5-{ts:F0}.pdf");
        }
    }
}

PDF used for testing test.pdf

Output would become this without this PR: out2-1645190757122.pdf (password 1234)