处理304错误号有bug，会崩溃

[ComradeFu]

原因是 api/Etag 的处理没有 return true;
导致 res.end() 两次（一次在 Etag 的handle里，一次在外面的调用）。

The reason is that the api/Etag handling does not return true;
resulting in res.end() being called twice (once in the handle of the Etag and once outside).

[dermasmid]

the error is
Error [ERR_STREAM_WRITE_AFTER_END]: write after end