Enable computeChecksums instead of providing ContentMD5

[iam7seven7]

#1 I encounter the same problem when the file size is larger than 5MB. aws-cli will automatically use multi-part upload. And because the request data is chucked, the MD5 is not valid. There's an issue for it at aws-cli repo and the official suggestion is to let aws-cli calculation the hash for you.

https://github.com/aws/aws-sdk-js/issues/774#issuecomment-155837412