Static: returning 404 when "index" is a non-regular file.

Before this patch, if "index" was a file, but not a regular file
nor a directory, so it may have been for example a FIFO, Unit
returned 404.  But if "index" was a directory, Unit returned 301.

For consistency, this patch makes Unit return 404 for every
non-regular file, including directories.

src/nxt_http_static.c

@@ -571,7 +571,9 @@ nxt_http_static_send_ready(nxt_task_t *task, void *obj, void *data)
         /* Not a file. */
         nxt_file_close(task, f);
 
-        if (nxt_slow_path(!nxt_is_dir(&fi))) {
+        if (nxt_slow_path(!nxt_is_dir(&fi)
+                          || shr->start[shr->length - 1] == '/'))
+        {
             nxt_log(task, NXT_LOG_ERR, "\"%FN\" is not a regular file",
                     f->name);