HTTP Compliance: entry is stale if request has max-age=0.

We should always do validation for requests with Cache-Control max-age=0,
even when entry age is also zero. In our case, RFC 2616 says:
    freshness_lifetime = max_age_value
    response_is_fresh = (freshness_lifetime > current_age)
and response_is_fresh is always false if freshness_lifetime is zero.

The check code was introduced in r5998 with a "Import of fix-ranges
branch" message. The code was commented out at the time of that
commit, for reasons unknown.

Test case:
    test_case/rfc2616/noSrv-hit-stale-max-age-req

=== modified file 'src/refresh.cc'
--- src/refresh.cc	2010-05-14 09:47:03 +0000
+++ src/refresh.cc	2010-09-16 02:34:29 +0000
@@ -298,48 +298,45 @@ refreshCheck(const StoreEntry * entry, H
             /* The clients no-cache header is ignored */
             debugs(22, 3, "refreshCheck: MAYBE: ignore-reload");
         } else if (R->flags.reload_into_ims || Config.onoff.reload_into_ims) {
             /* The clients no-cache header is changed into a IMS query */
             debugs(22, 3, "refreshCheck: YES: reload-into-ims");
             return STALE_RELOAD_INTO_IMS;
         } else {
             /* The clients no-cache header is not overridden on this request */
             debugs(22, 3, "refreshCheck: YES: client reload");
             request->flags.nocache = 1;
             return STALE_FORCED_RELOAD;
         }
 
 #endif
         if (NULL != cc) {
             if (cc->max_age > -1) {
 #if USE_HTTP_VIOLATIONS
                 if (R->flags.ignore_reload && cc->max_age == 0) {} else
 #endif
                 {
-#if 0
-
                     if (cc->max_age == 0) {
                         debugs(22, 3, "refreshCheck: YES: client-max-age = 0");
                         return STALE_EXCEEDS_REQUEST_MAX_AGE_VALUE;
                     }
 
-#endif
                     if (age > cc->max_age) {
                         debugs(22, 3, "refreshCheck: YES: age > client-max-age");
                         return STALE_EXCEEDS_REQUEST_MAX_AGE_VALUE;
                     }
                 }
             }
 
             if (EBIT_TEST(cc->mask, CC_MAX_STALE) && staleness > -1) {
                 if (cc->max_stale < 0) {
                     /* max-stale directive without a value */
                     debugs(22, 3, "refreshCheck: NO: max-stale wildcard");
                     return FRESH_REQUEST_MAX_STALE_ALL;
                 } else if (staleness < cc->max_stale) {
                     debugs(22, 3, "refreshCheck: NO: staleness < max-stale");
                     return FRESH_REQUEST_MAX_STALE_VALUE;
                 }
             }
         }
     }