Fix mail header encoding issues with MimeKit

[chenseanxy]

Fixes #441, #317

Following recommendations in MimeKit's FAQ:

.NET Core (and ASP.NET Core by extension) and .NET 5 only provide the Unicode encodings, ASCII and ISO-8859-1 by default. Other text encodings are not available to your application unless your application registers the encoding provider that provides all of the additional encodings.
First, add a package reference for the System.Text.Encoding.CodePages nuget package to your project and then register the additional text encodings using the following code snippet:

System.Text.Encoding.RegisterProvider (System.Text.CodePagesEncodingProvider.Instance);

Note: The above code snippet should be safe to call in .NET Framework versions >= 4.6 as well.

[chenseanxy]

Not sure if this is the best place for this though, ideally this should be invoked before any Mailkit calls