Remove setting senton (#142)

* Always set Representing

It seems that it is necessary to set the Representing properties all
the time, regardless of whether the message is being sent on behalf of
someone else.  Outlook seems to be expecting these fields to be
present.  If they are not, the From field in the folder messages lists
will be blank. (Note, if you pop up the message, the from fields will
be filled out.)

* Fix missing delivery time

* Only cast valid email addresses to MailboxAddress

Otherwise, the attempted cast with throw an InvalidCastException, in
certain situations, which I came across.

* Remove Setting SentOn = DateTime.UtcNow

It is better to leave this not set rather then setting the time to now,
as it is too assuming about the time of the message.  I came across
this processing emails from 12 years ago and they were showing up as
being created today.  So, setting the time to now, would be incorrect
and just confusing.  Better to leave it not set, as it will indicate
incomplete information.

* Remove Last NewLine To Match Base

Author: jamesjohnmcguire

MsgKit/Email.cs

@@ -478,8 +478,6 @@ internal void WriteToStorage()
         if (MessageEditorFormat != MessageEditorFormat.EDITOR_FORMAT_DONTKNOW)
             TopLevelProperties.AddProperty(PropertyTags.PR_MSG_EDITOR_FORMAT, MessageEditorFormat);
 
-        SentOn ??= DateTime.UtcNow;
-
         if (ReceivedOn.HasValue && ReceivedOn > DateTime.MinValue)
             TopLevelProperties.AddProperty(PropertyTags.PR_MESSAGE_DELIVERY_TIME, ReceivedOn.Value.ToUniversalTime());