String#to_a doesn't work in Ruby 1.9

In Ruby 1.8, counterintuitively, "foo".to_a would return ["foo"] - an
array with a single entry, the string. This went away in Ruby 1.9, likely
as a transition path to a much less confusing set of semantics.

Either way we should stop relying on that behaviour and get with the program.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>