Fix issue with Puppet

Puppet gem (under `Ruby 1.8`) overrides `Object#instance_variables` method.
It returns `Array` of `Symbol` rather than `Array` of `String`.
There is no `Symbol#downcase` under `Ruby 1.8`.

All of this reasons cause `NameError` when `ls` is invoked.

https://github.com/puppetlabs/puppet/blob/master/lib/puppet/util/monkey_patches.rb#L278