itunes: support lowercased episodeType values (#53)

GitHub: fix GH-51

The specification doesn't use lowercased values but Apple uses lowercased values internally.
See also: #51 (comment)

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Co-authored-by: Artem Alimov <liberlanco@gmail.com>

Author: ozydingo

lib/rss/rss.rb

@@ -593,7 +593,7 @@ def itunes_episode_type_writer(name, disp_name=name)
       module_eval(<<-DEF, *get_file_and_line_from_caller(2))
         def #{name}=(new_value)
           if @do_validate and
-               !["Full", "Trailer", "Bonus", nil].include?(new_value)
+               !["Full", "full", "Trailer", "trailer", "Bonus", "bonus", nil].include?(new_value)
             raise NotAvailableValueError.new('#{disp_name}', new_value)
           end
           @#{name} = new_value

test/test-itunes.rb

@@ -456,6 +456,8 @@ def assert_itunes_episodeType(readers, &rss20_maker)
       _wrap_assertion do
         assert_equal("Trailer",
                      set_itunes_episodeType("Trailer", readers, &rss20_maker))
+        assert_equal("trailer",
+                     set_itunes_episodeType("trailer", readers, &rss20_maker))
         assert_raise(NotAvailableValueError.new("episodeType", "Unknown")) do
           set_itunes_episodeType("Unknown", readers, &rss20_maker)
         end