How to set the header for a html email?

In El Capitan, the support for “set html content to myhtml” seem to be not functioning correctly. When I do that in the script below, the myhtml is included with the email but the header of the email message say “Content-Type: text/plain; charset=us-ascii” rather then "Content-Type: multipart/alternative; boundary=“Apple-Mail=_B09C9239-C6AF-4DD3-9B79-671528EAE1E1"”. The result is the the receiver can not see the html content.

Maybe I can set the header by manually, but I’m having trouble with the syntax. Please advice.

set toName to ""
set toAddress to ""
set textSubject to "HTML Test3"
set myheader to "Content-Type: multipart/alternative; boundary=\"Apple-Mail=_B09C9239-C6AF-4DD3-9B79-671528EAE1E1\""
set textBody to "<HTML>
<BODY bgcolor=\"#99CCFF\">
        <H1>Hi There</H1>
        <p>This is very minimal <u>\"hello world\"</u> HTML document.</p>
mailMessageCreate(toName, toAddress, textSubject, textBody)
on mailMessageCreate(toName, toAddress, textSubject, textBody)
            tell application "Mail"
  set refMessage to make new outgoing message with properties {subject:textSubject, visible:false} -- visible MUST be false to work
  tell refMessage
                 set html content to textBody -- must set the _HTML CONTENT_ rather than the _CONTENT_
                      make new to recipient at end of to recipients with properties {name:toName, address:toAddress}
                      --Set new to receipient to header with properties myheader (..or something like that)
                      -- send
   end tell
  end tell
end mailMessageCreate