External/Contextual serializers are not supported

Description

External and Contextual serializers registered via SerialModule are ignored at the moment.

(https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/custom_serializers.md#registering-and-context)

Issue

Seems like serializer is obtained using the serializerByTypeToken(type) method, which is not aware of registered serial modules. https://github.com/JakeWharton/retrofit2-kotlinx-serialization-converter/blob/d7d74722e8e5adf05f716d2d6e524ee34ce611f1/src/main/java/com/jakewharton/retrofit2/converter/kotlinx/serialization/Factory.kt#L23

Not sure if it’s feasible to fix it in a reasonably clean manner without breaking changes but as quick check this line makes the test pass:

    val loader = (serializer as FromString).format.context.getContextualOrDefault((type as Class<*>).kotlin)

To Reproduce

Failing Test (following existing deserialize() test):

    data class User(val name: String)

    object UserSerializer : KSerializer<User> {
        override val descriptor = PrimitiveDescriptor("User", STRING)

        override fun deserialize(decoder: Decoder): User =
                UserResponse.serializer().deserialize(decoder).run {
                    User(name = name)
                }

        override fun serialize(encoder: Encoder, value: User): Unit = TODO()

        @Serializable
        private data class UserResponse(val name: String)
    }

    @Before
    fun setUp() {
        val contentType = MediaType.get("application/json; charset=utf-8")
        val retrofit = Retrofit.Builder()
                .baseUrl(server.url("/"))
                .addConverterFactory(Json {
                    serialModule = serializersModuleOf(User::class, UserSerializer)
                }.asConverterFactory(contentType))
                .build()
        service = retrofit.create(Service::class.java)
    }

    @Test
    fun deserialize() {
        server.enqueue(MockResponse().setBody("""{"name":"Bob"}"""))
        val user = service.deserialize().execute().body()!!
        assertEquals(User("Bob"), user)
    }

[sgarfinkel]

Good question. I should have some time to look into the changes to make sure this feature can be implemented. If so I’ll try and get a PR pushed.

[JakeWharton]

Released in 0.7.0