Skip cannot Encode optional dictionary

[rafaelnobrekz]

Hello! When using JSONEncoder, dictionary types that are optional are not correctly encoded by Skip.

struct TestEntity2: Codable {
    let optionalDict: [String: String]?
    let nonOptionalDict: [String: String]
}

// This succeeds ✅
try! JSONEncoder().encode(TestEntity2(optionalDict: nil, nonOptionalDict: [:]))
// This fails ❌ data error skip.lib.ErrorException: java.lang.ClassCastException: skip.lib.Tuple2 cannot be cast to skip.lib.Encodable
try! JSONEncoder().encode(TestEntity2(optionalDict: ["1": "abc"], nonOptionalDict: ["1": "abc"]))

I'm currently working around by using a non optional dict and making it empty to represent nil.
Thank you!