Encoding should use document encoding, rather than UTF-8 all the time

[clementinelove]

FeedKit/Sources/FeedKit/Parser/XMLFeedParser.swift

Line 198 in fb75f2a

guard let string = String(data: CDATABlock, encoding: .utf8) else {

contents of this XML document shouldn't always be treated as UTF-8 Encoding, instead it should be decoded according to the encoding specified by the document, or, perhaps let the FeedParser accept a encoding argument.