Make sure serializing axlsx packages with identical contents always results in identical zip files.

This improves the possibilites for caching and/or consolidating the generated zip (xlsx) files.

Up to now, serializing the same package at different times resulted in different zip files because of the timestamp in the zip entry metadata.

Note: To generate identical packages (and thus identical zip files), you'll have set Core#created explicitly, eg. with `Package.new(created_at: Time.local(2013, 1, 1)`.