ceph s3 compatible api, multipart complete fail

[coztest]

I use this go s3 library for ceph s3 api. But when I finish multipart upload using "func (m *Multi) Complete(parts []Part)", s3 responsed http code 400. And python boto is OK for ceph s3 api. So I used wireshark to capture http packet from boto and goamz，boto has a "Content-Length" in request header, and aws's document also has a "Content-Length", header,
http://docs.aws.amazon.com/zh_cn/AmazonS3/latest/API/mpUploadComplete.html. Finally I added the header for goamz, Complete(parts []Part) running well, and I think aws s3 is also compatible too. #260