Fix issue with registering default polymorphic serializer

[piotrtomiak]

It looks like the code was copied and the update deserializer -> serializer was not made in that single place.

The following would throw an exception (but shouldn't):

SerializersModule {
    polymorphicDefaultDeserializer(AutomationActivityV1::class) { AutomationActivityV1Serializer }
    polymorphicDefaultSerializer(AutomationActivityV1::class) { AutomationActivityV1Serializer }
}

whereas the following would not (but should):

SerializersModule {
    polymorphicDefaultSerializer(AutomationActivityV1::class) { AutomationActivityV1Serializer }
    polymorphicDefaultSerializer(AutomationActivityV1::class) { AutomationActivityV1Serializer }
}

[sandwwraith]

It's already fixed in dev: #1849

[piotrtomiak]

Sorry, wrongly assumed that master is the development branch.