Simplify access to response body in WebClient

This commit makes a change to WebClient in oder to facilitate getting
the response body as a `Mono<Object>` or `Flux<Object>` without having
to deal with `ClientResponse`.

Specifically, this commit:

 - Adds `RequestHeaderSpec.retrieve` methods, next to `exchange`, that
 return the response body (and not a `ClientResponse`). Two convenience
 methods return the response body as `Mono` or `Flux`.
 - Adds ClientResponse.toRequestEntity to convert the ClientResponse
 into a RequestEntity.

Issue: SPR-15294

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

@@ -133,6 +133,50 @@ public class WebClientIntegrationTests {
 		Assert.assertEquals("application/json", recordedRequest.getHeader(HttpHeaders.ACCEPT));
 	}
 
+	@Test
+	public void jsonStringRetrieveMono() throws Exception {
+		String content = "{\"bar\":\"barbar\",\"foo\":\"foofoo\"}";
+		this.server.enqueue(new MockResponse().setHeader("Content-Type", "application/json")
+				.setBody(content));
+
+		Mono<String> result = this.webClient.get()
+				.uri("/json")
+				.accept(MediaType.APPLICATION_JSON)
+				.retrieveMono(String.class);
+
+		StepVerifier.create(result)
+				.expectNext(content)
+				.expectComplete()
+				.verify(Duration.ofSeconds(3));
+
+		RecordedRequest recordedRequest = server.takeRequest();
+		Assert.assertEquals(1, server.getRequestCount());
+		Assert.assertEquals("/json", recordedRequest.getPath());
+		Assert.assertEquals("application/json", recordedRequest.getHeader(HttpHeaders.ACCEPT));
+	}
+
+	@Test
+	public void jsonStringRetrieveFlux() throws Exception {
+		String content = "{\"bar\":\"barbar\",\"foo\":\"foofoo\"}";
+		this.server.enqueue(new MockResponse().setHeader("Content-Type", "application/json")
+				.setBody(content));
+
+		Flux<String> result = this.webClient.get()
+				.uri("/json")
+				.accept(MediaType.APPLICATION_JSON)
+				.retrieveFlux(String.class);
+
+		StepVerifier.create(result)
+				.expectNext(content)
+				.expectComplete()
+				.verify(Duration.ofSeconds(3));
+
+		RecordedRequest recordedRequest = server.takeRequest();
+		Assert.assertEquals(1, server.getRequestCount());
+		Assert.assertEquals("/json", recordedRequest.getPath());
+		Assert.assertEquals("application/json", recordedRequest.getHeader(HttpHeaders.ACCEPT));
+	}
+
 	@Test
 	public void jsonPojoMono() throws Exception {
 		this.server.enqueue(new MockResponse().setHeader("Content-Type", "application/json")