Permit calculated x-amz-content-sha256

[kdiogenes]

First, thanks for the lib, great work!

I'm using it to sign a multipart upload direct to S3, so the payload never hit my server. I'm calculating it in JavaScript and send it to my server to generate the authorization header.

I was thinking in augment AWSAuth.Utils.build_canonical_request to also check for :hashed in the hashed_payload, in this case, the function uses the hashed payload in the header, what do you think? Can I make a PR?

Best regards!

[kdiogenes]

I already opened a PR with a possible solution: #38