title: Configured ObjectMapper not used in spring-boot-webflux
body: <p>I have mixins configured in my objectmapperbuilder config, using the regular spring web controller, the data outputted according to the mixins.
  However using webflux, a controller with a method returning a Flow or Mono have the data serialized like if the objectmapper a default one.</p>
  
  <p>How to get webflux to enforce an objectmapper configuration to be used ?</p>
  
  <p>sample config:</p>
  
  <pre><code>@Bean
  JavaTimeModule javatimeModule(){
      return new JavaTimeModule();
  }
  
  @Bean
  Jackson2ObjectMapperBuilderCustomizer jackson2ObjectMapperBuilderCustomizer(){
  return jacksonObjectMapperBuilder -&gt;  jacksonObjectMapperBuilder.featuresToEnable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS)
                                                                      .mixIn(MyClass.class, MyClassMixin.class);
  }
  </code></pre>