Normalize x-amz-copy-source
Normalize x-amz-copy-source

Fixes https://github.com/jamhall/s3rver/issues/30

Since S3's documentation is ambiguous, `s3rver` should expect both versions of `x-amz-copy-source`. This change adds a '/' to `copy` in case `x-amz-copy-source` does not have a leading '/'.

For S3's documentation, see https://docs.aws.amazon.com/AmazonS3/latest/API/RESTObjectCOPY.html
rockmacaca/s3rver,jamhall/s3rver,jamhall/s3rver