MessageDispositionNotification ContentTransferEncoding ignored when added to MultipartSigned

[ProH4Ck]

Describe the bug
I have a problem with a MessageDispositionNotification added to a MultipartSigned. When I create the MDN (Message Disposition Notification) I specify ContentTransferEncoding = ContentEncoding.SevenBit but when I try to sign the message, MDN content is converted to a base64 string. This is a problem because my AS2 partner need to receive this part in plain-text

Platform (please complete the following information):

• OS: Windows
• .NET Runtime: NET 5
• MimeKit Version: 2.10.0

To Reproduce

public static async Task Main(string[] args)
{
    var multipartReport = new MultipartReport("disposition-notification");

    var textPart = new TextPart
    {
        ContentTransferEncoding = ContentEncoding.SevenBit,
        Content = new MimeContent(
            new MemoryStream(Encoding.UTF8.GetBytes("The AS2 message has been received.")))
    };

    var hl = new HeaderList
    {
        new Header("Reporting-UA", "AS2_Server"),
        new Header("Original-Recipient", "rfc822; AS2_SENDER"),
        new Header("Final-Recipient", "rfc822; AS2_SENDER"),
        new Header("Original-Message-ID", "TEST_MESSAGE"),
        new Header("Disposition", "automatic-action/MDN-sent-automatically; processed"),
        new Header("Received-Content-MIC", "<<MIC>>, sha1")
    };
    var mdnContent = new MemoryStream();
    await hl.WriteToAsync(mdnContent);
    var reportPart = new MessageDispositionNotification
    {
        Content = new MimeContent(mdnContent),
        ContentTransferEncoding = ContentEncoding.SevenBit
    };

    multipartReport.Add(textPart);
    multipartReport.Add(reportPart);

    Console.WriteLine("UNSIGNED MDN: " + Environment.NewLine + multipartReport);

    using var ctx = new TemporarySecureMimeContext();
    var signer = new CmsSigner("as2.pfx", "1234");
    var finalEntity = MultipartSigned.Create(ctx, signer, multipartReport);

    Console.WriteLine("SIGNED MDN: " + Environment.NewLine + finalEntity.ToString());
}

Expected behavior
The MDN is left in plain-text

Screenshots

[jstedfast]

Released v2.10.1 to nuget.org with this fix.