S3Proxy 2.3.0

• Allow x-amz-user-agent header
• Add middleware to override storage class
• Adds feature allowing Access-Control-Expose-Headers configuration
• Configure maximum-timeskew with environment variable
• Fix time skew validation for presigned urls
• Look up HTTP headers while ignoring case
• Remove timestamp validation for requests that use query string authentication

Thanks @losfair, @sullis, @timuralp, @timursaikaliev, @twickline, and @vilmosnagy for opening pull request to improve S3Proxy!