Cache: fixed caching of intercepted errors (ticket #1382).

When caching intercepted errors, previous behaviour was to use
proxy_cache_valid times specified, regardless of various cache control
headers present in the response.  Fix is to check u->cacheable and
use u->cache->valid_sec as set by various cache control response headers,
similar to how we do this in the normal caching code path.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index b3580fe..985d633 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -2528,13 +2528,23 @@
 #if (NGX_HTTP_CACHE)
 
             if (r->cache) {
-                time_t  valid;
 
-                valid = ngx_http_file_cache_valid(u->conf->cache_valid, status);
+                if (u->cacheable) {
+                    time_t  valid;
 
-                if (valid) {
-                    r->cache->valid_sec = ngx_time() + valid;
-                    r->cache->error = status;
+                    valid = r->cache->valid_sec;
+
+                    if (valid == 0) {
+                        valid = ngx_http_file_cache_valid(u->conf->cache_valid,
+                                                          status);
+                        if (valid) {
+                            r->cache->valid_sec = ngx_time() + valid;
+                        }
+                    }
+
+                    if (valid) {
+                        r->cache->error = status;
+                    }
                 }
 
                 ngx_http_file_cache_free(r->cache, u->pipe->temp_file);