(#6830) Fix case where instance_variables returns symbols in Ruby 1.9

class Foo
  def initialize
    @a = 2
    @b = "two"
  end
end

p Foo.new.instance_variables

In Ruby 1.8 this prints

["@a", "@b"]

In Ruby 1.9 this prints

[:@a, :@b]

Reviewed-by: Jesse Wolfe <jesse@puppetlabs.com>