Don't look for expiry-date when parsing headers of object with in-progress restore

Currently, if you try to request data about an object with an in-progress restore, you get an error when Ruby tries to parse a nil value as a DateTime. This happens because the headers sent from AWS for the object do not include an expiry-date value in the x-amz-restore header when the object is being restored.

This commit fixes the logic so it will fall back to just looking for the ongoing-request value if it can't find an expire-date in the header.