Container signature update depend on Last-Modified

[rhubner]

Container signature update is unable to work without Last-Modified http header. This header is not mandatory in HTTP response and in modern REST architecture is usually replaced with ETag

https://github.com/digital-preservation/droid/blob/master/droid-container/src/main/java/uk/gov/nationalarchives/droid/container/httpservice/ContainerSignatureHttpService.java#L121