Missing `Content-Length` in headers may cause nginx 411 Length Required Error.

I'm not sure this is a bug, but `content-length` should be prepared in headers so that server knows when the chunk datas end.

I fixed this in my own project about uploading a file by calling `getLength()` in headers, but this cause a lot time when sending a big file.

As I know, chrome added `content-length` before sending the whole post content.
