EncodableDictionary.swift

struct AnyEncodable: Encodable {
    let base: Encodable
    
    init(_ value: Encodable) {
        base = value
    }
    
    func encode(to encoder: Encoder) throws {
        try base.encode(to: encoder)
    }
}

struct EncodableDictionary: Encodable {
    private(set) var base: [String: AnyEncodable] = [:]
    
    subscript(key: String) -> Encodable? {
        get { return base[key]?.base }
        set { base[key] = newValue.map(AnyEncodable.init) }
    }
    
    func encode(to encoder: Encoder) throws {
        try base.encode(to: encoder)
    }
}

extension EncodableDictionary: ExpressibleByDictionaryLiteral {
    init(dictionaryLiteral elements: (String, Encodable?)...) {
        for case let (key, value?) in elements {
            base[key] = AnyEncodable(value)
        }
    }
}