rss: Accept empty text element as valid element

	Parser has been accepted it but XML serializer wasn't accepted.

	Reported by stefano frabetti. Thanks!!!

	[ruby-core:80965] [Bug #13531]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@59396 b2dd03c8-39d4-4d8f-98ff-823fe69b080e