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