rgw bugfix：s3 aws v2 signature priority between header['X-Amz-Date'] and header['Date']
as http://docs.aws.amazon.com/AmazonS3/latest/dev/RESTAuthentication.html Time Stamp Requirement section metioned
When an x-amz-date header is present in a request, the system will ignore any Date header when computing the request signature. Therefore, if you include the x-amz-date header, use the empty string for the Date when constructing the StringToSign
and in real life, some client mechine's time is shift to rgw server, and those mechine's time is not able to adjust, but we can set X-Amz-Date in request header to fix this problem, but in current version code, the header['Date'] priority is higher than header['X-Amz-Date'].
the PR https://github.com/ceph/ceph/pull/15467 fix this problem