Fix problem with 8bit content-transfer-encoding in nndoc mbox files

* lisp/gnus/nndoc.el (nndoc-possibly-change-buffer): If we're
reading an mbox file, it may contain messages that use
content-transfer-encoding 8bit, which means that we have to treat
the file as a sequence of byte (bug#42951).  This avoids
double-decoding -- once by Emacs when inserting the mbox into the
buffer, and once by Gnus when displaying the articles.

lisp/gnus/nndoc.el

@@ -352,6 +352,7 @@ from the document.")
 	    nndoc-group-alist)
       (setq nndoc-dissection-alist nil)
       (with-current-buffer nndoc-current-buffer
+	(set-buffer-multibyte nil)
 	(erase-buffer)
 	(condition-case error
 	    (if (and (stringp nndoc-address)