Exception occur when fetching with the "PreviewText" item

[moyanming]

Hi,
If the "| MessageSummaryItems.PreviewText" add to the fetch, there may be exception occur. It seems that when the preview text is null and the MimeUtils.TryParse() can't deal with this situation.

if ((client.Capabilities & (ImapCapabilities.SpecialUse | ImapCapabilities.XList)) != 0)
{
    var folder = client.GetFolder(SpecialFolder.Sent);
    folder.Open(FolderAccess.ReadOnly);
    var sums = folder.Fetch(0, -1, MessageSummaryItems.All
        | MessageSummaryItems.BodyStructure
        | MessageSummaryItems.PreviewText);
}

{"Value cannot be null.\r\nParameter name: text"}
at MimeKit.Utils.MimeUtils.TryParse(String text, ContentEncoding& encoding)\r\n   
at MailKit.Net.Imap.ImapFolder.FetchPreviewTextContext.Add(Section section)\r\n   
at MailKit.Net.Imap.ImapFolder.FetchStreamAsync(ImapEngine engine, ImapCommand ic, Int32 index, Boolean doAsync)\r\n   
at MailKit.Net.Imap.ImapEngine.ProcessUntaggedResponseAsync(Boolean doAsync, CancellationToken cancellationToken)\r\n   
at MailKit.Net.Imap.ImapCommand.StepAsync(Boolean doAsync)\r\n   
at MailKit.Net.Imap.ImapEngine.IterateAsync(Boolean doAsync)\r\n   
at MailKit.Net.Imap.ImapEngine.RunAsync(ImapCommand ic, Boolean doAsync)\r\n   
at MailKit.Net.Imap.ImapFolder.GetPreviewTextAsync(FetchSummaryContext sctx, Boolean doAsync, CancellationToken cancellationToken)\r\n   
at MailKit.Net.Imap.ImapFolder.FetchAsync(Int32 min, Int32 max, MessageSummaryItems items, Boolean doAsync, CancellationToken cancellationToken)\r\n   
at MailKit.Net.Imap.ImapFolder.Fetch(Int32 min, Int32 max, MessageSummaryItems items, CancellationToken cancellationToken)\r\n 

[jstedfast]

Thanks for the bug report!