Fix #1558 - add opts.extra_headers in 304 responses

mongoose.c

@@ -1445,7 +1445,8 @@ void mg_http_serve_file(struct mg_connection *c, struct mg_http_message *hm,
              (inm = mg_http_get_header(hm, "If-None-Match")) != NULL &&
              mg_vcasecmp(inm, etag) == 0) {
     mg_fs_close(fd);
-    mg_printf(c, "HTTP/1.1 304 Not Modified\r\nContent-Length: 0\r\n\r\n");
+    mg_printf(c, "HTTP/1.1 304 Not Modified\r\n%sContent-Length: 0\r\n\r\n",
+              opts->extra_headers ? opts->extra_headers : "");
   } else {
     int n, status = 200;
     char range[100] = "";

src/http.c

@@ -451,7 +451,8 @@ void mg_http_serve_file(struct mg_connection *c, struct mg_http_message *hm,
              (inm = mg_http_get_header(hm, "If-None-Match")) != NULL &&
              mg_vcasecmp(inm, etag) == 0) {
     mg_fs_close(fd);
-    mg_printf(c, "HTTP/1.1 304 Not Modified\r\nContent-Length: 0\r\n\r\n");
+    mg_printf(c, "HTTP/1.1 304 Not Modified\r\n%sContent-Length: 0\r\n\r\n",
+              opts->extra_headers ? opts->extra_headers : "");
   } else {
     int n, status = 200;
     char range[100] = "";

test/unit_test.c

@@ -655,6 +655,7 @@ static void test_http_server(void) {
 
   {
     extern char *mg_http_etag(char *, size_t, size_t, time_t);
+    struct mg_http_message hm;
     char etag[100];
     size_t size = 0;
     time_t mtime = 0;
@@ -662,6 +663,9 @@ static void test_http_server(void) {
     ASSERT(mg_http_etag(etag, sizeof(etag), size, mtime) == etag);
     ASSERT(fetch(&mgr, buf, url, "GET /a.txt HTTP/1.0\nIf-None-Match: %s\n\n",
                  etag) == 304);
+    mg_http_parse(buf, strlen(buf), &hm);
+    ASSERT(mg_http_get_header(&hm, "A") != NULL);
+    ASSERT(mg_strcmp(*mg_http_get_header(&hm, "A"), mg_str("B")) == 0);
   }
 
   // Text mime type override