Bug fixes and feature enhancements for S3.

1. AWS::S3::Client :copy_object : clear 'transfer-encoding' req header.
2. AWS::S3::Request : add 'content_type' attr_accessor.
3. AWS::S3::S3Object#signature : override content-type, 'x-amz-storage-class' defaults with request values.
4. AWS::S3::S3Object#url_for : add options for content_type, storage_class.