Fix GH-1725

<itunes:explicit> accepts "explicit", "yes", "true", "clean", "no" and
"false" as valid values.

Here is the <itunes:explicit>'s description in
https://help.apple.com/itc/podcasts_connect/#/itcb54353390:

> The <itunes:explicit> tag indicates whether your podcast contains
> explicit material. You can specify the following values:
>
>   * Yes | Explicit | True. If you specify yes, explicit, or true,
>     indicating the presence of explicit content, the iTunes Store
>     displays an Explicit parental advisory graphic for your podcast.
>
>   * Clean | No | False. If you specify clean, no, or false, indicating
>     that none of your podcast episodes contain explicit language or
>     adult content, the iTunes Store displays a Clean parental
>     advisory graphic for your podcast.

I don't know whether <itunes:explicit> value is case sensitive or
insensitive. But the current implementation is case insensitive.

Reported by Valerie Woolard Srinivasan. Thanks!!!


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60367 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

When you change this to true, you may need to add more tests.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53141 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

  [Fixes GH-485] https://github.com/ruby/ruby/pull/485
* ext/openssl/ossl_config.c: ditto
* lib/rss/utils.rb, lib/time.rb: ditto
* test/ruby/envutil.rb: ditto


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  - 0.1.9 -> 0.2.0.
  - supported Slash module.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@13446 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

  - 0.1.7 -> 0.1.8.
  - supported <itunes:XXX>.
  - reverted backward incompatibility API changes introduced 0.1.7.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@12871 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

  - supported Atom.
  - bumped version 0.1.6 to 0.1.7.
* sample/rss/convert.rb: added new sample.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@12087 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

* lib/rss/parser.rb: added class name registry for complex model
  elements. (ex. have childlen elements, have some attributes and
  a child element and so on.)

* lib/rss/maker/base.rb: added default current_element implementation.

* lib/rss/maker/dublincore.rb: supported multiple DublinCore items.

* lib/rss/maker/image.rb: supproted new DublinCore API.

* lib/rss/trackback.rb (RSS::TrackBackUtils.new_with_value_if_need):
  moved to RSS::Utils.

* lib/rss/utils.rb (RSS::Utils.new_with_value_if_need):
  moved from RSS::TrackBackUtils.


* lib/rss/maker/image.rb: fixed invalid argument of
  add_need_initialize_variable bug.

* lib/rss/maker/trackback.rb: ditto.


* lib/rss/rss.rb (Hash#merge): added for ruby 1.6.

* lib/rss/rss.rb (RSS::BaseModel.date_writer): changed to accept nil
  for date value.

* test/test_dublincore.rb: added tests for plural accessor and
  multiple DublinCore items.

* test/test_setup_maker_1.0.rb: fixed swapped actual and expected
  values.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@8252 b2dd03c8-39d4-4d8f-98ff-823fe69b080e