Allow x-amz-content-sha256 to be optional for PutObject() (#5340)

x-amz-content-sha256 can be optional for any AWS signature v4
requests, make sure to skip sha256 calculation when payload
checksum is not set.

Here is the overall expected behavior

** Signed request **
- X-Amz-Content-Sha256 is set to 'empty' or some 'value'  or its
  not 'UNSIGNED-PAYLOAD'- use it to validate the incoming payload.
- X-Amz-Content-Sha256 is set to 'UNSIGNED-PAYLOAD' - skip checksum verification
- X-Amz-Content-Sha256 is not set we use emptySHA256

** Presigned request **
- X-Amz-Content-Sha256 is set to 'empty' or some 'value'  or its
  not 'UNSIGNED-PAYLOAD'- use it to validate the incoming payload
- X-Amz-Content-Sha256 is set to 'UNSIGNED-PAYLOAD' - skip checksum verification
- X-Amz-Content-Sha256 is not set we use 'UNSIGNED-PAYLOAD'

Fixes #5339

cmd/object-handlers.go

@@ -585,6 +585,7 @@ func (api objectAPIHandlers) PutObjectHandler(w http.ResponseWriter, r *http.Req
 			writeErrorResponse(w, s3Err, r.URL)
 			return
 		}
+
 	case authTypePresigned, authTypeSigned:
 		if s3Err = reqSignatureV4Verify(r, globalServerConfig.GetRegion()); s3Err != ErrNone {
 			errorIf(errSignatureMismatch, "%s", dumpRequest(r))