Add body(Object) method to ServerResponse.BodyBuilder

This method introduces a new body(Object) to ServerResponse, a shortcut
to body(BodyInserters.fromObject(Object)).

Note that in the implementation of the method, an `instanceof` check is
performed to make sure that the passed argument is not a `Publisher`,
as users should call `body(Publisher, Class)` for sending a reactive
type.

This Publisher-check is also done in the `WebClient`, for the same
reasons.

Issue: SPR-15461

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultWebClientTests.java

@@ -116,6 +116,14 @@ public class DefaultWebClientTests {
 		verifyNoMoreInteractions(this.exchangeFunction);
 	}
 
+	@Test(expected = IllegalArgumentException.class)
+	public void bodyObjectPublisher() throws Exception {
+		Mono<Void> mono = Mono.empty();
+		WebClient client = builder().build();
+
+		client.post().uri("http://example.com").body(mono);
+	}
+
 
 	private WebClient.Builder builder() {
 		return WebClient.builder().baseUrl("/base").exchangeFunction(this.exchangeFunction);

spring-webflux/src/test/java/org/springframework/web/reactive/function/server/DefaultServerResponseBuilderTests.java

@@ -35,9 +35,8 @@ import org.springframework.http.MediaType;
 import org.springframework.mock.http.server.reactive.test.MockServerHttpResponse;
 import org.springframework.web.server.ServerWebExchange;
 
-import static org.junit.Assert.assertEquals;
-import static org.mockito.Mockito.mock;
-import static org.mockito.Mockito.when;
+import static org.junit.Assert.*;
+import static org.mockito.Mockito.*;
 
 /**
  * @author Arjen Poutsma
@@ -295,4 +294,12 @@ public class DefaultServerResponseBuilderTests {
 		StepVerifier.create(response.getBody()).expectComplete().verify();
 	}
 
+	@Test(expected = IllegalArgumentException.class)
+	public void bodyObjectPublisher() throws Exception {
+		Mono<Void> mono = Mono.empty();
+
+		ServerResponse.ok().body(mono);
+	}
+
+
 }

spring-webflux/src/test/java/org/springframework/web/reactive/function/server/NestedRouteIntegrationTests.java

@@ -25,7 +25,6 @@ import org.springframework.http.ResponseEntity;
 import org.springframework.web.client.RestTemplate;
 
 import static org.junit.Assert.*;
-import static org.springframework.web.reactive.function.BodyInserters.fromObject;
 import static org.springframework.web.reactive.function.server.RequestPredicates.GET;
 import static org.springframework.web.reactive.function.server.RequestPredicates.path;
 import static org.springframework.web.reactive.function.server.RouterFunctions.nest;
@@ -82,11 +81,11 @@ public class NestedRouteIntegrationTests extends AbstractRouterFunctionIntegrati
 	private static class NestedHandler {
 
 		public Mono<ServerResponse> bar(ServerRequest request) {
-			return ServerResponse.ok().body(fromObject("bar"));
+			return ServerResponse.ok().body("bar");
 		}
 
 		public Mono<ServerResponse> baz(ServerRequest request) {
-			return ServerResponse.ok().body(fromObject("baz"));
+			return ServerResponse.ok().body("baz");
 		}
 
 		public Mono<ServerResponse> variables(ServerRequest request) {