DOn't use string-as-unibyte in Gnus

DOn't use string-as-unibyte in Gnus

* lisp/gnus/nnmail.el (nnmail-parse-active): Don't use
string-as-unibyte.
(nnmail-insert-xref): Ditto.

* lisp/gnus/canlock.el (canlock-make-cancel-key): Ditto.

* lisp/gnus/gnus-art.el (gnus-article-browse-html-parts): Ditto.

* lisp/gnus/gnus-srvr.el (gnus-browse-foreign-server): Ditto.
(gnus-browse-foreign-server): Ditto.
(gnus-browse-foreign-server): Ditto.

* lisp/gnus/gnus-start.el
(gnus-update-active-hashtb-from-killed): Ditto.
(gnus-read-newsrc-el-file): Ditto.

* lisp/gnus/mml.el (mml-generate-mime-1): Ditto.

* lisp/gnus/nnir.el (nnir-get-active): Ditto.
(nnir-get-active): Ditto.