"stg mail" doesn't set Content-Type and such

As can be seen in the headers of the stgit patches I've send to this
list recently, "stgit mail" doesn't seem to handle charsets and stuff
when sending mail; from what I can tell, it just sends the raw bytes
without setting Content-Transfer-Encoding, Content-Type, or anything
similar.

-- 
Karl Hasselström, kha@treskal.com
      www.treskal.com/kalle