merge fastcgi_cache_key

src/http/modules/ngx_http_fastcgi_module.c

diff --git a/src/http/modules/ngx_http_fastcgi_module.c b/src/http/modules/ngx_http_fastcgi_module.c
index 773707d..dfa5346 100644
--- a/src/http/modules/ngx_http_fastcgi_module.c
+++ b/src/http/modules/ngx_http_fastcgi_module.c
@@ -2064,6 +2064,10 @@
     ngx_conf_merge_ptr_value(conf->upstream.cache_valid,
                              prev->upstream.cache_valid, NULL);
 
+    if (conf->cache_key.value.data == NULL) {
+        conf->cache_key = prev->cache_key;
+    }
+
 #endif
 
     ngx_conf_merge_value(conf->upstream.pass_request_headers,