itunes: support for lowercased episodeType values

[liberlanco]

I met this issue with following RSS: https://feeds.fireside.fm/smartlogic/rss

They uses lowercase values for episodeType like 'trailer' or 'full'. I tried 2 different RSS readers with the URL and all of them read this feed without problems (liferea and some from flatpack, forget the name).

ruby/rss raises exception:

RSS::NotAvailableValueError: value <trailer> of tag <episodeType> is not available. (RSS::NotAvailableValueError)

I also tried RSS validator, and it gave many errors but not about episodeType value: https://www.rssboard.org/rss-validator/check.cgi?url=https%3A%2F%2Ffeeds.fireside.fm%2Fsmartlogic%2Frss

Hence I understood that official specs defines Full, Trailer and Bonus as legal values for this XML tag, but looks like it's already common practice to use lowercase values (even validator doesn't think it's bad)

So here is the patch to support them + small test update with lowercase value

[kou]

I can understand what you think but please disable validation for this case. (And please feedback this to fireside.fm if you want.)
If we provide a "loose" validation, users can't trust validated feeds. Users need some workarounds even with validated feeds. For example, users need to normalize case for this case.

In general, feed readers parse feed loosely as much as possible because there are many broken feeds in the world and feed readers needs to process them as much as possible.

[kou]

Could you accept only Full/full variants? (We don't want to accept FULL, fULL and so on.

Suggested change

	!["full","trailer","bonus",nil].include?(new_value.downcase)
	!["Full","full","Trailer","trailer","Bonus","bonus",nil].include?(new_value)

[kou]

Apple uses full/trailer/bonus too internally.
See also: #51 (comment)

[kou]

Implemented by #53.