Add constructor to TestRestTemplate that takes a RestTemplateBuilder

Closes gh-6706
See gh-6702

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

@@ -82,6 +82,19 @@ public class TestRestTemplate {
 
 	private final RestTemplate restTemplate;
 
+	/**
+	 * Create a new {@link TestRestTemplate} instance.
+	 * @param restTemplateBuilder builder used to configure underlying {@link RestTemplate}
+	 */
+	public TestRestTemplate(RestTemplateBuilder restTemplateBuilder) {
+		this(buildRestTemplate(restTemplateBuilder));
+	}
+
+	private static RestTemplate buildRestTemplate(RestTemplateBuilder restTemplateBuilder) {
+		Assert.notNull(restTemplateBuilder, "RestTemplateBuilder must not be null");
+		return restTemplateBuilder.build();
+	}
+
 	/**
 	 * Create a new {@link TestRestTemplate} instance.
 	 * @param httpClientOptions client options to use if the Apache HTTP Client is used

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

@@ -24,6 +24,7 @@ import org.junit.Test;
 
 import org.springframework.boot.test.web.client.TestRestTemplate.CustomHttpComponentsClientHttpRequestFactory;
 import org.springframework.boot.test.web.client.TestRestTemplate.HttpClientOption;
+import org.springframework.boot.web.client.RestTemplateBuilder;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;
 import org.springframework.http.client.InterceptingClientHttpRequestFactory;
@@ -33,6 +34,7 @@ import org.springframework.web.client.RestOperations;
 import org.springframework.web.client.RestTemplate;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.given;
 import static org.mockito.Mockito.mock;
 
 /**
@@ -43,6 +45,15 @@ import static org.mockito.Mockito.mock;
  */
 public class TestRestTemplateTests {
 
+	@Test
+	public void fromRestTemplateBuilder() {
+		RestTemplateBuilder builder = mock(RestTemplateBuilder.class);
+		RestTemplate delegate = new RestTemplate();
+		given(builder.build()).willReturn(delegate);
+		assertThat(new TestRestTemplate(builder).getRestTemplate())
+				.isEqualTo(delegate);
+	}
+
 	@Test
 	public void simple() {
 		// The Apache client is on the classpath so we get the fully-fledged factory