S3 spec does not accept an ILM XML document containing both <Filter>
and <Prefix> XML tags, even if both are empty. That is why we added
a 'set' field in some lifecycle structures to decide when and when not to
show a tag. However, we forgot to disallow marshaling of Filter when
'set' is set to false.
This will fix ILM document replication in a site replication
configuration in some cases.