WebClient supports Void response body

WebResponseExtractor uses Mono.when with the response status, headers,
and the decoded body. However when the response body stream is empty
then when completes empty too.

This change adds defaultIfEmpty handling for en empty response body.

spring-web-reactive/src/main/java/org/springframework/web/client/reactive/WebResponseExtractors.java

@@ -42,6 +42,9 @@ public class WebResponseExtractors {
 
 	private static final Object[] HINTS = new Object[] {UTF_8};
 
+	private static final Object EMPTY_BODY = new Object();
+
+
 	/**
 	 * Extract the response body and decode it, returning it as a {@code Mono<T>}
 	 */
@@ -68,18 +71,21 @@ public class WebResponseExtractors {
 	 * Extract the full response body as a {@code ResponseEntity}
 	 * with its body decoded as a single type {@code T}
 	 */
-	public static <T> WebResponseExtractor<Mono<ResponseEntity<T>>> response(Class<T> sourceClass) {
+	public static <T> WebResponseExtractor<Mono<ResponseEntity<T>>> response(Class<T> bodyClass) {
 
-		ResolvableType resolvableType = ResolvableType.forClass(sourceClass);
+		ResolvableType bodyType = ResolvableType.forClass(bodyClass);
 		return webResponse -> webResponse.getClientResponse()
-				.then(response ->
-						Mono.when(
-								decodeResponseBody(response, resolvableType, webResponse.getMessageDecoders()).next(),
+				.then(response -> {
+					List<Decoder<?>> decoders = webResponse.getMessageDecoders();
+					return Mono.when(
+							decodeResponseBody(response, bodyType, decoders).next().defaultIfEmpty(EMPTY_BODY),
 							Mono.just(response.getHeaders()),
-								Mono.just(response.getStatusCode())))
+							Mono.just(response.getStatusCode()));
+				})
 				.map(tuple -> {
+					Object body = (tuple.getT1() != EMPTY_BODY ? tuple.getT1() : null);
 					//noinspection unchecked
-					return new ResponseEntity<>((T) tuple.getT1(), tuple.getT2(), tuple.getT3());
+					return new ResponseEntity<>((T) body, tuple.getT2(), tuple.getT3());
 				});
 	}