* GH-3483: Fallback to RestTemplate.converters
Fixes https://github.com/spring-projects/spring-integration/issues/3483
When there is no reasonable way to determine a `Content-Type`
from the request message, do not set an `application/x-java-serialized-object`
as a fallback and let the `RestTemplate` to determine the target type and
conversion through its `HttpMessageConverter` set
* Remove `application/x-java-serialized-object` fallback from the
`AbstractHttpRequestExecutingMessageHandler`
* Adjust its log messages according `LogAccessor`
* Un`@Disable` `WebFluxDslTests` since fix was done in Spring Security
* Add `HttpRequestExecutingMessageHandlerTests.testNoContentTypeAndSmartConverter()`
* Mention change in `What's New`
* Fix language in whats-new.adoc
Co-authored-by: Gary Russell <grussell@vmware.com>
Co-authored-by: Gary Russell <grussell@vmware.com>
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