Upstream: fixed inheritance of proxy_store and friends.

The proxy_store, fastcgi_store, scgi_store and uwsgi_store were inherited
incorrectly if a directive with variables was defined, and then redefined
to the "on" value, i.e. in configurations like:

    proxy_store /data/www$upstream_http_x_store;

    location / {
        proxy_store on;
    }
diff --git a/src/http/modules/ngx_http_fastcgi_module.c b/src/http/modules/ngx_http_fastcgi_module.c
index 8520c00..1c63786 100644
--- a/src/http/modules/ngx_http_fastcgi_module.c
+++ b/src/http/modules/ngx_http_fastcgi_module.c
@@ -2432,7 +2432,7 @@
     ngx_hash_init_t               hash;
     ngx_http_core_loc_conf_t     *clcf;
 
-    if (conf->upstream.store != 0) {
+    if (conf->upstream.store == NGX_CONF_UNSET) {
         ngx_conf_merge_value(conf->upstream.store,
                               prev->upstream.store, 0);
 
diff --git a/src/http/modules/ngx_http_proxy_module.c b/src/http/modules/ngx_http_proxy_module.c
index f04436d..c3105e2 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -2624,7 +2624,7 @@
     ngx_http_proxy_rewrite_t   *pr;
     ngx_http_script_compile_t   sc;
 
-    if (conf->upstream.store != 0) {
+    if (conf->upstream.store == NGX_CONF_UNSET) {
         ngx_conf_merge_value(conf->upstream.store,
                               prev->upstream.store, 0);
 
diff --git a/src/http/modules/ngx_http_scgi_module.c b/src/http/modules/ngx_http_scgi_module.c
index c6e3b88..c8cf9aa 100644
--- a/src/http/modules/ngx_http_scgi_module.c
+++ b/src/http/modules/ngx_http_scgi_module.c
@@ -1186,7 +1186,7 @@
     ngx_hash_init_t               hash;
     ngx_http_core_loc_conf_t     *clcf;
 
-    if (conf->upstream.store != 0) {
+    if (conf->upstream.store == NGX_CONF_UNSET) {
         ngx_conf_merge_value(conf->upstream.store, prev->upstream.store, 0);
 
         if (conf->upstream.store_lengths == NULL) {
diff --git a/src/http/modules/ngx_http_uwsgi_module.c b/src/http/modules/ngx_http_uwsgi_module.c
index ae069eb..8e269d7 100644
--- a/src/http/modules/ngx_http_uwsgi_module.c
+++ b/src/http/modules/ngx_http_uwsgi_module.c
@@ -1400,7 +1400,7 @@
     ngx_hash_init_t               hash;
     ngx_http_core_loc_conf_t     *clcf;
 
-    if (conf->upstream.store != 0) {
+    if (conf->upstream.store == NGX_CONF_UNSET) {
         ngx_conf_merge_value(conf->upstream.store, prev->upstream.store, 0);
 
         if (conf->upstream.store_lengths == NULL) {