* lisp/gnus: Remove assumptions about mail-header being a vector

* lisp/gnus: Remove assumptions about mail-header being a vector

Use `mail-header-p` and `make-full-mail-header` instead of `vectorp` and
`vector`, respectively.

* lisp/gnus/message.el (message-cite-original-1, message-reply)
(message-followup):
* lisp/gnus/gnus-sum.el (gnus-get-newsgroup-headers):
Use make-full-mail-header instead of `vector`.
(gnus--dummy-mail-header): New const, using make-full-mail-header.
(gnus-update-summary-mark-positions): Use it instead of
an immediate vector.
(gnus-set-mode-line, gnus-summary-article-pseudo-p)
(gnus-summary-article-subject, gnus-summary-insert-subject)
(gnus-summary-find-subject, gnus-summary-goto-subject)
(gnus-summary-limit-to-age, gnus-summary-find-matching)
(gnus-summary-mark-article-as-unread, gnus-summary-mark-article)
(gnus-summary-save-article):
* lisp/gnus/gnus.el (gnus-news-group-p): Use mail-header-p instead
of vectorp.