Headers filter: fixed "add_header ... '' always".

The "always" parameter was ignored if the header value was empty.
diff --git a/src/http/modules/ngx_http_headers_filter_module.c b/src/http/modules/ngx_http_headers_filter_module.c
index 985e5b3..6738afe 100644
--- a/src/http/modules/ngx_http_headers_filter_module.c
+++ b/src/http/modules/ngx_http_headers_filter_module.c
@@ -712,17 +712,17 @@
 
     if (value[2].len == 0) {
         ngx_memzero(&hv->value, sizeof(ngx_http_complex_value_t));
-        return NGX_CONF_OK;
-    }
 
-    ngx_memzero(&ccv, sizeof(ngx_http_compile_complex_value_t));
+    } else {
+        ngx_memzero(&ccv, sizeof(ngx_http_compile_complex_value_t));
 
-    ccv.cf = cf;
-    ccv.value = &value[2];
-    ccv.complex_value = &hv->value;
+        ccv.cf = cf;
+        ccv.value = &value[2];
+        ccv.complex_value = &hv->value;
 
-    if (ngx_http_compile_complex_value(&ccv) != NGX_OK) {
-        return NGX_CONF_ERROR;
+        if (ngx_http_compile_complex_value(&ccv) != NGX_OK) {
+            return NGX_CONF_ERROR;
+        }
     }
 
     if (cf->args->nelts == 3) {