Remove use of WebClient#exchange from WebTestClient

The exchange() method is now deprecated because it is not safe for
general use but that doesn't apply to the WebTestClient because it
exposes a different higher level API for response handling that
ensures the response is consumed. Nevertheless WebTestClient cannot
call WebClient.exchange() any more.

To fix this WebTestClient no longer delegates to WebClient and thus
gains direct access to the underlying ExchangeFunction. This is
not a big deal because WebClient and WebTestClient do the same
only when it comes to gathering builder options and request input.

See gh-25751

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

@@ -43,11 +43,11 @@ import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.BDDMockito.given;
+import static org.mockito.BDDMockito.when;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.verifyNoInteractions;
 import static org.mockito.Mockito.verifyNoMoreInteractions;
-import static org.mockito.Mockito.when;
 
 /**
  * Unit tests for {@link DefaultWebClient}.

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/client/WebClientExtensionsTests.kt

@@ -80,6 +80,7 @@ class WebClientExtensionsTests {
 	}
 
 	@Test
+	@Suppress("DEPRECATION")
 	fun awaitExchange() {
 		val response = mockk<ClientResponse>()
 		every { requestBodySpec.exchange() } returns Mono.just(response)