Allow to specify the content-type for the copy-object operation.

This is needed both for preserving the content type of the object being copied, as well as to change the content-type if desired. If the content type is not specified, the copied object gets the conetnt type "binary/octet-stream".
Merge pull request #123 from romanb/s3-copy-object-content-type

Allow to specify the content-type for the copy-object operation.