Contextual serializer is ignored in serializerOrNull function

[rsinukov]

Describe the bug

To Reproduce

    @Test
    fun testContextualByType() {
        val json = Json { serializersModule = serializersModuleOf(UserData::class, UserSerializer) }
        val type = typeOf<UserData>()
        val serializer = json.serializersModule.serializerOrNull(type)
        assertEquals(UserSerializer, serializer)
    }

    @Serializable
    public data class UserData(val id: Int, val name: String)

    public object UserSerializer : KSerializer<UserData> {
        override val descriptor: SerialDescriptor = buildClassSerialDescriptor("UserData")
        override fun deserialize(decoder: Decoder): UserData = TODO()
        override fun serialize(encoder: Encoder, value: UserData) = TODO()
    }   

Expected behavior
The test should pass.

I believe that the problem is in these lines inside serializerByKTypeImpl function.

    val result: KSerializer<Any>? = when {
        typeArguments.isEmpty() -> rootClass.serializerOrNull() ?: getContextual(rootClass)
        else -> builtinSerializer(typeArguments, rootClass, failOnMissingTypeArgSerializer)
    }?.cast()

The expression rootClass.serializerOrNull() ?: getContextual(rootClass) should be the opposite: first try to get contextual and if it's not found, fallback to default serializer.

Environment

• Kotlin version: [e.g. 1.6.10]
• Library version: [e.g. 1.3.0]

[sandwwraith]

I think we won't change the semantics of the existing stable function in such an unpredictable way for users (see also #2026). Whether we should provide a new function that prioritizes module over built-in is an open question.