Problem with url-retrieve when HTTP status is 304

[Mark Plaksin]

With CVS Emacs, url-retrieve triggers an error when it gets an HTTP status
of 304 *and* the requested page is not in the cache.  To reproduce, try
this:

(setq url-request-extra-headers
'(("If-Modified-Since" . "Sun, 18 Sep 2005 17:34:28 GMT")
  ("If-None-Match" . "\"91ad777b5809a56e2b2b9d7c4833fceb\"")))
(url-retrieve-synchronously "http://tumbleblindly.org/feed/";)

You should get an error that the cache file doesn't exist.  (If the feed
changes after I send this note, you'll have to replace the date with the
value of the the feed's Last-Modified header and the long string with the
value of the ETag header.)

It's not clear which should change--the section of url-http-parse-headers
which handles 304 or url-cache-extract.  One of them should check to see
whether the cache file exists before trying to insert its contents.

Here's the relevant section of url-http-parse-headers:

           (304
            ;; The 304 response MUST NOT contain a message-body.
            (url-http-debug "Extracting document from cache... (%s)"
                            (url-cache-create-filename (url-view-url t)))
            (url-cache-extract (url-cache-create-filename (url-view-url t)))
            (setq redirect-uri nil
                  success t))

And here's url-cache-extract:

(defun url-cache-extract (fnam)
  "Extract FNAM from the local disk cache"
  (erase-buffer)
  (insert-file-contents-literally fnam))