* Add the support for a `Publisher`, `Resource` and `MultiValueMap`
into the `WebFluxRequestExecutingMessageHandler`
* Along side with the `WebFluxRequestExecutingMessageHandler.setPublisherElementType`
and `WebFluxRequestExecutingMessageHandler.setPublisherElementTypeExpression`,
add XSD support for the `publisher-element-type(-expression)`, which
is used for the element type when request body is a `Publisher`
* Polishing for `AbstractHttpRequestExecutingMessageHandler`
* Fix Sonar smells for affected classes
* Remove used imports
Doc polishing
f8f69c9129