(message-header-hook): Change default valiable to `nil' instead of

(message-header-hook): Change default valiable to `nil' instead of
`'(eword-decode-header)'. Update doc string.

(message-encode-header-function): New variable.
(message-after-header-encode-hook): New variable.
(message-send-mail): Funcall `message-encode-header-function' after run
`message-header-hook'.
And run hooks `message-after-header-encode-hook'.
(message-send-news): Ditto.
(message-do-fcc): Ditto.