quoted-printable encoder generates lines longer than 76 characters

[georg87]

Describe the bug
The quoted-printable encoder generates lines longer than the allowed 76 characters in case '=' is the 76th sign of a line. The '=' sign is encoded with '=3D' and the line becomes 78 characters long.

Platform (please complete the following information):

• OS: Windows, Linux (Docker)
• .NET Runtime: CoreCLR
• .NET Framework: .NET 6.0
• MimeKit Version: 3.1.1 (MimeKitLite)

Expected behavior
The quoted-printable encoder generates lines which are no longer than 76 characters.

Code Snippets

MimeKit.MimeMessage msg = new MimeKit.MimeMessage();
MimeKit.BodyBuilder builder = new MimeKit.BodyBuilder();
builder.HtmlBody = "<table style=\"width:100%;\" cellpadding=\"0\" cellspacing=\"0\" border=\"0\"><tr><td style=\"width:100%;text-align:center;background-color:;\" bgcolor=\"\">Test</td></tr><table>";
msg.Body = builder.ToMessageBody();
// force quoted-printable encoding for this example
(msg.BodyParts.First() as MimeKit.MimePart).ContentTransferEncoding = MimeKit.ContentEncoding.QuotedPrintable;

Console.WriteLine(msg.Body.ToString());

/*

Output (second line is too long):

Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable

<table style=3D"width:100%;" cellpadding=3D"0" cellspacing=3D"0" border=3D"0=
"><tr><td style=3D"width:100%;text-align:center;background-color:;" bgcolor=3D=
"">Test</td></tr><table>

*/