bug#15097: 24.3.50; json.el can't encode lists of lists

[Rolando Pereira]

Hello all,

The function `json-encode' can't encode a list thats composed only of other
lists, i.e. the following doesn't work:

    (json-encode '((1 2 3))) => Error: (json-key-format 1)

If I had an extra "nil" in the topmost list `json-encode' then it works
fine:

    (json-encode '((1 2 3) nil)) => "[[1,2,3],null]"

However if I replace the `nil' with another list then I get the same
error:

    (json-encode '((1 2 3) (4))) => Error: (json-key-format 1)

Here's a small ERT test that should trigger the bug:

    (ert-deftest json-encode-list-of-lists-test ()
      (ert-should (string= (json-encode '(1 2 3)) "[1,2,3]"))              ; 
works
      (ert-should (string= (json-encode '((1) 2 3)) "[[1],2,3]"))          ; 
works
      (ert-should (string= (json-encode '((1 2 3))) "[[1,2,3]]"))          ; 
doesn't work
      (ert-should (string= (json-encode '((1 2 3) nil)) "[[1,2,3],null]")) ; 
works
      (ert-should (string= (json-encode '((1 2 3) (4))) "[[1,2,3],[4]]"))  ; 
doesn't work
      )


Best regards,
Rolando Pereira