[FIX] Make the `print_value` method work with `false` values
[FIX] Make the `print_value` method work with `false` values

Prevent values with a value set to `false` being skipped; eg. setting the
`node.bootstrap.mlockall = false` wouldn't print the line in the YAML
configuration file, due to the `||=` semantics.

This patch now checks the `nil` value explicitely.

Related: #39

gilt/cookbook-elasticsearch,gouketsu/cookbook-elasticsearch,gilt/cookbook-elasticsearch,nbudin/cookbook-elasticsearch,zzugg/cookbook-elasticsearch,gouketsu/cookbook-elasticsearch,vacay/vcy-elasticsearch,vacay/vcy-elasticsearch,spuder/cookbook-elasticsearch,gouketsu/cookbook-elasticsearch,gilt/cookbook-elasticsearch,GemHQ/cookbook-elasticsearch,zzugg/cookbook-elasticsearch,michaeld/elasticsearch-cookbook,bavardage/cookbook-elasticsearch,paulczar/cookbook-elasticsearch,spuder/cookbook-elasticsearch,zzugg/cookbook-elasticsearch,vacay/vcy-elasticsearch,nbudin/cookbook-elasticsearch,spuder/cookbook-elasticsearch,nbudin/cookbook-elasticsearch