Content-Type is not replaced when updating source object on Copy Object [JIRA: RCS-287]

[ksauzz]

Addressed by #1246

[Basho-JIRA]

For release note: prior 2.1.0, PUT Copy API with identical source and destination changes user metadata (x-amz-meta-* headers) but fails to update Content-Type. This fixes Content-Type is correctly updated by the API call.

_[posted via JIRA by Shunichi Shinohara]_