Document how to to customize the TestRestTemplate

Update the reference documentation and add some additional Javadoc to
provide hints on how to customize the `TestRestTemplate`.

Fixes gh-6465

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -5468,6 +5468,40 @@ public class MyTest {
 }
 ----
 
+If you are using the `@SpringBootTest` annotation, you can just inject a fully configured
+`TestRestTemplate` and start usinging it. If necessary, additional customizations can be
+applied via the `RestTemplateBuilder` bean:
+
+[source,java,indent=0]
+----
+@RunWith(SpringRunner.class)
+@SpringBootTest
+public class MyTest {
+
+	@Autowired
+	private TestRestTemplate template;
+
+	@Test
+	public void testRequest() throws Exception {
+		HttpHeaders headers = template.getForEntity("http://myhost.com", String.class).getHeaders();
+		assertThat(headers.getLocation().toString(), containsString("myotherhost"));
+	}
+
+	@TestConfig
+	static class Config {
+
+		@Bean
+		public RestTemplateBuilder restTemplateBuilder() {
+			return new RestTemplateBuilder()
+				.additionalMessageConverters(...)
+				.customizers(...);
+		}
+
+	}
+
+}
+----
+
 
 
 [[boot-features-websockets]]

spring-boot-test/src/main/java/org/springframework/boot/test/context/SpringBootTest.java

@@ -54,6 +54,8 @@ import org.springframework.web.context.WebApplicationContext;
  * including the ability to start a fully running container listening on a
  * {@link WebEnvironment#DEFINED_PORT defined} or {@link WebEnvironment#RANDOM_PORT
  * random} port.</li>
+ * <li>Registers a {@link org.springframework.boot.test.web.client.TestRestTemplate
+ * TestRestTemplate} bean for use in web tests.</li>
  * </ul>
  *
  * @author Phillip Webb

spring-boot-test/src/main/java/org/springframework/boot/test/web/client/TestRestTemplate.java

@@ -36,6 +36,7 @@ import org.apache.http.impl.client.HttpClients;
 import org.apache.http.protocol.HttpContext;
 import org.apache.http.ssl.SSLContextBuilder;
 
+import org.springframework.boot.web.client.RestTemplateBuilder;
 import org.springframework.core.ParameterizedTypeReference;
 import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpHeaders;
@@ -66,6 +67,12 @@ import org.springframework.web.util.UriTemplateHandler;
  * Note: To prevent injection problems this class internally does not extend
  * {@link RestTemplate}. If you need access to the underlying {@link RestTemplate} use
  * {@link #getRestTemplate()}.
+ * <p>
+ * If you are using the
+ * {@link org.springframework.boot.test.context.SpringBootTest @SpringBootTest}
+ * annotation, a {@link TestRestTemplate} is automatically available and can be
+ * {@code @Autowired} into you test. If you need customizations (for example to adding
+ * additional message converters) use a {@link RestTemplateBuilder} {@code @Bean}.
  *
  * @author Dave Syer
  * @author Phillip Webb