Extend handling of "Cache-Control" header.

* module/web/http.scm ("Cache-Control"): Value for `max-stale' is
  optional.  Strict validation for value-less directives (`no-store',
  etc.).  String values optional for "cache-extension" directives.
* test-suite/tests/web-http.test: Value for `max-stale' is optional.

module/web/http.scm

@@ -1240,19 +1240,25 @@ phrase\"."
 (declare-key-value-list-header! "Cache-Control"
   (lambda (k v-str)
     (case k
-      ((max-age max-stale min-fresh s-maxage)
+      ((max-age min-fresh s-maxage)
        (parse-non-negative-integer v-str))
+      ((max-stale)
+       (and v-str (parse-non-negative-integer v-str)))
       ((private no-cache)
        (and v-str (split-header-names v-str)))
       (else v-str)))
   (lambda (k v)
     (case k
-      ((max-age max-stale min-fresh s-maxage)
+      ((max-age min-fresh s-maxage)
        (non-negative-integer? v))
+      ((max-stale)
+       (or (not v) (non-negative-integer? v)))
       ((private no-cache)
        (or (not v) (list-of-header-names? v)))
+      ((no-store no-transform only-if-cache must-revalidate proxy-revalidate)
+       (not v))
       (else
-       (not v))))
+       (or (not v) (string? v)))))
   (lambda (k v port)
     (cond
      ((string? v) (display v port))

test-suite/tests/web-http.test

@@ -83,6 +83,8 @@
                  '((private . (foo))))
   (pass-if-parse cache-control "no-cache,max-age=10"
                  '(no-cache (max-age . 10)))
+  (pass-if-parse cache-control "max-stale" '(max-stale))
+  (pass-if-parse cache-control "max-stale=10" '((max-stale . 10)))
 
   (pass-if-parse connection "close" '(close))
   (pass-if-parse connection "Content-Encoding" '(content-encoding))