Update single character representation test for Ruby 2.

In about_strings.rb, neither of the `test_in_ruby_*_single_characters_are_represented_by_*` tests applied to Ruby 2. Update `test_in_ruby_1_9_single_characters_are_represented_by_strings ` to handle Ruby 2 and rename to `test_in_ruby_1_9_and_2_single_characters_are_represented_by_strings`.