set charset for a single email message

[Secret Guy]

Hi,

I'm trying to learn thai, and I want to send a single email with
character encoding tis-620. I haven't yet tried setting
gnus-group-charset-alist, but I'm hoping there is a simpler way to
change the encoding for a single email.

I also tried adding:

  Content-Type: text/plain; charset=tis-620

but, gnus changes that to charset=utf-8.

Any advice? I'm using Gnu emacs 21.3.1 with mule.