Inserting headers at top causes change in header order

[nitinag]

When trying to insert a header on the top of the header list, the Content-* Body headers are moved to the top:

var sourceStream = new MemoryStream(Encoding.UTF8.GetBytes(@"From: Example Test <[email protected]>
MIME-Version: 1.0
Content-Type: multipart/mixed;
   boundary=""simple boundary""

Test
"));

var mimeMessage = MimeMessage.Load(sourceStream);
mimeMessage.Headers.Insert(0, HeaderId.Received, "Test");

The resulting output is:

Content-Type: multipart/mixed;
   boundary="simple boundary"
Received: Test
From: Example Test <[email protected]>
MIME-Version: 1.0

Test

Expected behavior
MimeKit should maintain header order and add the header to the top.

[jstedfast]

I've got a fix for this locally, but I want to verify with some unit tests first.

This week is back-to-back meetings, so I'm finding it difficult to make time to write code this week.

[nitinag]

No worries, thanks for the work addressing it!