Jackson serialization problem of entities using Groovy

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### Task List

- [x] Steps to reproduce provided
- [x] Stacktrace (if present) provided
- [x] Example that reproduces the problem uploaded to Github
- [x] Full description of the issue provided (see below)

### Steps to Reproduce

1. Create a Micronaut Data + Groovy project.
2. Create a class annotated with `@Entity` which has data structure properties (`List`, `Map`, etc.).
3. Create a controller endpoint which responds with an instance of the entity class.
4. Contact the endpoint.

### Expected Behaviour

The endpoint should serialize the entity instance with its data structure properties populated.

### Actual Behaviour

An `io.micronaut.http.codec.CodecException` exception is thrown upon trying to serialize an entity instance which has data structure properties populated:
```
object [example.domain.EntityClass@2c58806f] to JSON: java.lang.Integer cannot be cast to java.lang.String (through reference chain: example.domain.EntityClass["map"])
io.micronaut.http.codec.CodecException: Error encoding object [example.domain.EntityClass@2c58806f] to JSON: java.lang.Integer cannot be cast to java.lang.String (through reference chain: example.domain.EntityClass["map"])
at io.micronaut.jackson.codec.JacksonMediaTypeCodec.encode(JacksonMediaTypeCodec.java:215)
at io.micronaut.jackson.codec.JacksonMediaTypeCodec.encode(JacksonMediaTypeCodec.java:221)
at io.micronaut.http.server.netty.RoutingInBoundHandler.encodeBodyAsByteBuf(RoutingInBoundHandler.java:1405)
at io.micronaut.http.server.netty.RoutingInBoundHandler.encodeBodyWithCodec(RoutingInBoundHandler.java:1351)
at io.micronaut.http.server.netty.RoutingInBoundHandler.lambda$subscribeToResponsePublisher$12(RoutingInBoundHandler.java:1279)
at io.reactivex.internal.operators.flowable.FlowableMap$MapSubscriber.onNext(FlowableMap.java:63)
at io.reactivex.internal.operators.flowable.FlowableSwitchMap$SwitchMapSubscriber.drain(FlowableSwitchMap.java:306)
at io.reactivex.internal.operators.flowable.FlowableSwitchMap$SwitchMapInnerSubscriber.onSubscribe(FlowableSwitchMap.java:365)
at io.micronaut.reactive.rxjava2.RxInstrumentedSubscriber.onSubscribe(RxInstrumentedSubscriber.java:56)
at io.reactivex.internal.operators.flowable.FlowableJust.subscribeActual(FlowableJust.java:34)
at io.reactivex.Flowable.subscribe(Flowable.java:14935)
at io.reactivex.Flowable.subscribe(Flowable.java:14882)
at io.micronaut.reactive.rxjava2.RxInstrumentedFlowable.subscribeActual(RxInstrumentedFlowable.java:58)
at io.reactivex.Flowable.subscribe(Flowable.java:14935)
at io.reactivex.Flowable.subscribe(Flowable.java:14882)
at io.reactivex.internal.operators.flowable.FlowableSwitchMap$SwitchMapSubscriber.onNext(FlowableSwitchMap.java:129)
at io.reactivex.internal.operators.flowable.FlowableSubscribeOn$SubscribeOnSubscriber.onNext(FlowableSubscribeOn.java:97)
at io.micronaut.reactive.rxjava2.RxInstrumentedSubscriber.onNext(RxInstrumentedSubscriber.java:70)
at io.reactivex.internal.operators.flowable.FlowableSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FlowableSwitchIfEmpty.java:59)
at io.micronaut.reactive.rxjava2.RxInstrumentedSubscriber.onNext(RxInstrumentedSubscriber.java:70)
at io.reactivex.internal.operators.flowable.FlowableMap$MapSubscriber.onNext(FlowableMap.java:68)
at io.micronaut.reactive.rxjava2.RxInstrumentedSubscriber.onNext(RxInstrumentedSubscriber.java:70)
at io.reactivex.internal.operators.flowable.FlowableCreate$NoOverflowBaseAsyncEmitter.onNext(FlowableCreate.java:403)
at io.micronaut.http.server.netty.RoutingInBoundHandler.lambda$buildResultEmitter$16(RoutingInBoundHandler.java:1482)
at io.reactivex.internal.operators.flowable.FlowableCreate.subscribeActual(FlowableCreate.java:71)
at io.reactivex.Flowable.subscribe(Flowable.java:14935)
at io.reactivex.internal.operators.flowable.FlowableMap.subscribeActual(FlowableMap.java:37)
at io.reactivex.Flowable.subscribe(Flowable.java:14935)
at io.reactivex.internal.operators.flowable.FlowableSwitchIfEmpty.subscribeActual(FlowableSwitchIfEmpty.java:32)
at io.reactivex.Flowable.subscribe(Flowable.java:14935)
at io.reactivex.Flowable.subscribe(Flowable.java:14882)
at io.micronaut.http.server.context.ServerRequestContextFilter$1.subscribe(ServerRequestContextFilter.java:66)
at io.reactivex.internal.operators.flowable.FlowableFromPublisher.subscribeActual(FlowableFromPublisher.java:29)
at io.reactivex.Flowable.subscribe(Flowable.java:14935)
at io.reactivex.Flowable.subscribe(Flowable.java:14882)
at io.reactivex.internal.operators.flowable.FlowableSubscribeOn$SubscribeOnSubscriber.run(FlowableSubscribeOn.java:82)
at io.reactivex.internal.schedulers.ExecutorScheduler$ExecutorWorker$BooleanRunnable.run(ExecutorScheduler.java:288)
at io.reactivex.internal.schedulers.ExecutorScheduler$ExecutorWorker.run(ExecutorScheduler.java:253)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: com.fasterxml.jackson.databind.JsonMappingException: java.lang.Integer cannot be cast to java.lang.String (through reference chain: example.domain.EntityClass["map"])
at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:397)
at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:356)
at com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:316)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:730)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:166)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:4110)
at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsBytes(ObjectMapper.java:3437)
at io.micronaut.jackson.codec.JacksonMediaTypeCodec.encode(JacksonMediaTypeCodec.java:212)
... 40 common frames omitted
Caused by: java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
at com.fasterxml.jackson.databind.ser.std.StringSerializer.isEmpty(StringSerializer.java:35)
at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeOptionalFields(MapSerializer.java:770)
at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:639)
at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:33)
at io.micronaut.jackson.modules.BeanIntrospectionModule$BeanIntrospectionPropertyWriter.serializeAsField(BeanIntrospectionModule.java:660)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:722)
```
The problem doesn't happen with Java. It looks similar to the one reported in this [issue](https://github.com/micronaut-projects/micronaut-core/issues/3861).

### Environment Information

- **Operating System**: Ubuntu 20.04
- **Micronaut Version:** 1.3.7
- **Micronaut Data Version:** 1.1.3
- **JDK Version:** 1.8
- **Groovy Version:** 2.5.8

### Example Application

- https://github.com/tcrespog/mn-data-serialization-problem (the `java` branch contains the working Java version of the application whereas the `master` branch contains the Groovy version).

Run the linked mn-data-serialization-problem application, comparing its Groovy master branch with the working Java branch and reproducing the endpoint response. Start with BeanIntrospectionModule.java and JacksonMediaTypeCodec.java from the stack trace; done means entity List and Map properties serialize without the CodecException.

Written by the indexing model from the issue text.