Mixed choice/non-choice decoding

[bwetherfield]

Mirrors #154

Fixes bugs

1. Decoding multiple choice elements in the same Keyed Container.
2. Decoding choice elements after decoding regular keyed elements in the same container.

Case 1 refers to decoding implementations of the form:

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        otherValue = try container.decode(String.self, forKey: .otherValue)
        intOrString = try IntOrString(from: decoder)
    }

where IntOrString is a choice coding element

(IntOrString defined as follows)

enum IntOrString {
    case int(Int)
    case string(String)
}

extension IntOrString: Decodable {
    enum CodingKeys: String, CodingKey {
        case int
        case string
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        if container.contains(.int) {
            self = .int(try container.decode(Int.self, forKey: .int))
        } else {
            self = .string(try container.decode(String.self, forKey: .string))
        }
    }
}

extension IntOrString.CodingKeys: XMLChoiceCodingKey {} // signifies that `IntOrString` is a choice element

Case 2 refers to decoding implementations of the following form:

    init(from decoder: Decoder) throws {
        self.first = try IntOrString(from: decoder)
        self.second = try AlternateIntOrString(from: decoder)
    }

Where both first: IntOrString and second: AlternateIntOrString are choice elements.

(AlternateIntOrString defined as follows)

private enum AlternateIntOrString {
    case alternateInt(Int)
    case alternateString(String)
}

extension AlternateIntOrString: Decodable {
    enum CodingKeys: String, CodingKey {
        case alternateInt = "alternate-int"
        case alternateString = "alternate-string"
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        if container.contains(.alternateInt) {
            self = .alternateInt(try container.decode(Int.self, forKey: .alternateInt))
        } else {
            self = .alternateString(try container.decode(String.self, forKey: .alternateString))
        }
    }
}

extension AlternateIntOrString.CodingKeys: XMLChoiceCodingKey {} // signifies that `AlternateIntOrString` is a choice element

[MaxDesiatov]

Seems legit 👍