email: When setting an 8bit CTE, remove the old header

email: When setting an 8bit CTE, remove the old header

From the docs [1]:

  Note that this does not overwrite or delete any existing header with
  the same name. If you want to ensure that the new header is the only
  one present in the message with field name name, delete the field
  first, e.g.:

    del msg['subject']
    msg['subject'] = 'Python roolz!'

[1]: http://docs.python.org/3/library/email.message.html#email.message.Message.__setitem__

Signed-off-by: W. Trevor King <wking@tremily.us>