TestRestTemplate should not override request factory

Previously `TestRestTemplate` would override the configured
`ClientHttpRequestFactory` if the Apache HTTP client library was on
classpath.

This commit fixes two issues:

1. The existing `ClientHttpRequestFactory` is overridden *only* if it is
using the Apache HTTP client variant, in order to wrap it with the
`TestRestTemplate` custom support

2. Calling `withBasicAuth` will no longer directly use the request
factory returned by the internal `RestTemplate`; if client interceptors
are configured, the request factory is wrapped with an
`InterceptingClientHttpRequestFactory`. If we don't unwrap it,
interceptors are copied/applied twice in the newly created
`TestRestTemplate` instance. For that, we need to use reflection as the
underlying request factory is not accessible directly.

Closes gh-8697

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/web/client/TestRestTemplateTests.java

@@ -37,6 +37,8 @@ import org.springframework.http.client.ClientHttpRequestFactory;
 import org.springframework.http.client.ClientHttpRequestInterceptor;
 import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
 import org.springframework.http.client.InterceptingClientHttpRequestFactory;
+import org.springframework.http.client.OkHttp3ClientHttpRequestFactory;
+import org.springframework.http.client.SimpleClientHttpRequestFactory;
 import org.springframework.http.client.support.BasicAuthorizationInterceptor;
 import org.springframework.mock.env.MockEnvironment;
 import org.springframework.mock.http.client.MockClientHttpRequest;
@@ -82,6 +84,15 @@ public class TestRestTemplateTests {
 				.isInstanceOf(HttpComponentsClientHttpRequestFactory.class);
 	}
 
+	@Test
+	public void doNotReplaceCustomRequestFactory() {
+		RestTemplateBuilder builder = new RestTemplateBuilder()
+				.requestFactory(OkHttp3ClientHttpRequestFactory.class);
+		TestRestTemplate testRestTemplate = new TestRestTemplate(builder);
+		assertThat(testRestTemplate.getRestTemplate().getRequestFactory())
+				.isInstanceOf(OkHttp3ClientHttpRequestFactory.class);
+	}
+
 	@Test
 	public void getRootUriRootUriSetViaRestTemplateBuilder() {
 		String rootUri = "http://example.com";
@@ -125,6 +136,7 @@ public class TestRestTemplateTests {
 	@Test
 	public void restOperationsAreAvailable() {
 		RestTemplate delegate = mock(RestTemplate.class);
+		given(delegate.getRequestFactory()).willReturn(new SimpleClientHttpRequestFactory());
 		given(delegate.getUriTemplateHandler())
 				.willReturn(new DefaultUriBuilderFactory());
 		RestTemplateBuilder builder = mock(RestTemplateBuilder.class);