CopyObject meta request bug fixes

Invoke headers callback when CopyObject request is bypassed

Prior to this fix the headers callback was invoked only
when the Copy meta request performed a multi-part copy, but not
when the object was small and resulted in a bypassed CopyObject request.

This commit fixes the issue, invoking the headers callback
in both cases.

Support leading slash in x-amz-copy-source

The CopyObject meta request required the x-amz-copy-source to be in strict form of {bucket}/{key}.
But as per S3 docs, /{bucket}/{key} is also valid.

This fix adds support for both formats.