Add ZookeeperConsumer#create_streams_by_filter

Decided on only supporting whitelist filters for now, as I don’t really
see the point in supporting blacklists (though I’m sure someone else
will), so until someone requests the functionality or submits a pull
request there will only be support for whitelists.
Fix incorrect tests for ZookeeperConsumer#create_streams

Didn’t actually test that Fixnum’s was explicitly converted to Java
integers.