Date formatter with fractional seconds handling from Codable structs

CodableDateFormatter.swift

extension Formatter {
    static let iso8601: (regular: ISO8601DateFormatter, withFractionalSeconds: DateFormatter) = {
        let formatter = DateFormatter()
        formatter.calendar = Calendar(identifier: .iso8601)
        formatter.locale = Locale(identifier: "en_US_POSIX")
        formatter.timeZone = TimeZone(secondsFromGMT: 0)
        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSXXXXX"
        return (ISO8601DateFormatter(), formatter)
    }()
}
extension JSONDecoder.DateDecodingStrategy {
    static let iso8601withFractionalSeconds = custom {
        let container = try $0.singleValueContainer()
        let string = try container.decode(String.self)
        if let date = Formatter.iso8601.withFractionalSeconds.date(from: string) {
            return date
        } else if let date = Formatter.iso8601.regular.date(from: string) {
            return date
        } else {
            throw DecodingError.dataCorruptedError(in: container, debugDescription: "Invalid date: " + string)
        }
    }
}
extension JSONEncoder.DateEncodingStrategy {
    static let iso8601withFractionalSeconds = custom {
        var container = $1.singleValueContainer()
        try container.encode(Formatter.iso8601.withFractionalSeconds.string(from: $0))
    }
}

Hey @Ikloo, thanks for publishing it! Do you think it makes sense to update it to the following since .withFractionalSeconds is supported since iOS 11:

static let iso8601: (regular: ISO8601DateFormatter, withFractionalSeconds: ISO8601DateFormatter) = {
    let formatter = ISO8601DateFormatter()
    formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
    return (ISO8601DateFormatter(), formatter)
  }()