ExchangeActions update

Put assertBodyXyz options behind a common assertBody() entry point
currently including "isEmpty" and "asMap" but in the future others
related to JSON content or XPath for example.

Now that ExchangeActions provides method to access the ExchangeInfo
it has been removed from constructors of assertion classes that
already have ExchangeActions.

spring-test/src/test/java/org/springframework/test/web/reactive/server/samples/ResponseEntityTests.java

@@ -95,7 +95,7 @@ public class ResponseEntityTests {
 				.exchange(Mono.just(new Person("John")), Person.class)
 				.assertStatus().isCreated()
 				.assertHeader("location").isEqualTo("/persons/John").and()
-				.assertBodyIsEmpty();
+				.assertBody().isEmpty();
 	}