bug#20972: 25.0.50; eieio-persistent broken

[Jan Tatarik <jan.tatarik@gmail.com>]

The commit a0010db41ca83a8211162b649e679162dd4153a6 has broken eieio
persistence.

Besides my own stuff, it affects gnus registry - just lost my registry
file, because gnus couldn't read the registry file, so it created a new,
empty one...

I found three issues, the recipe can be reproduced with emacs -Q:

(require 'eieio)
(require 'eieio-base)

(defclass test-persist (eieio-persistent)
  ((slot :initarg :slot :type string))
  "Test class.")

;; In theory, calling it like this should override the :file slot
;; defined in eieio-persistent
(eieio-persistent-save (test-persist :slot "foo") "/tmp/test.eieio")

;; But when no :file has been specified, it breaks
;; Debugger entered--Lisp error: (unbound-slot test-persist "#<test-persist test-persist>" file oref)

;; with explicit :file it works
(eieio-persistent-save (test-persist :file "/tmp/test.eieio" :slot "foo"))

;; But this stores the file in :file, no override takes place, contrary to the docs
(eieio-persistent-save (test-persist :file "/tmp/test.eieio" :slot "foo")
                        "/tmp/another_file.eieio")


;; And finally, the most important issue - we cannot read the files back in
(eieio-persistent-read "/tmp/test.eieio")
;; Debugger entered--Lisp error: (wrong-type-argument arrayp test-persist)


The first two issues (if they are issues), must have been already
present for some time.

The eieio-persistent-read was caused by the latest changes to eieio
(tested with 8bab1490f14207eeeee4b2f4ad30b5d695db8245 and it still
worked).