Prevent CastCastException when stripping root URI

Update RootUriRequestExpectationManager to only wrap requests when
they cannot be cast to MockClientHttpRequest. This prevents later
ClassCastExceptions from being thrown with @RestClientTests that
define expected body content.

Fixes gh-6845

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

@@ -42,4 +42,8 @@ public class ExampleRestClient {
 		return this.restTemplate.getForEntity("/test", String.class).getBody();
 	}
 
+	public void testPostWithBody(String body) {
+		this.restTemplate.postForObject("/test", body, String.class);
+	}
+
 }

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

@@ -25,6 +25,7 @@ import org.springframework.test.context.junit4.SpringRunner;
 import org.springframework.test.web.client.MockRestServiceServer;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.springframework.test.web.client.match.MockRestRequestMatchers.content;
 import static org.springframework.test.web.client.match.MockRestRequestMatchers.requestTo;
 import static org.springframework.test.web.client.response.MockRestResponseCreators.withSuccess;
 
@@ -57,4 +58,11 @@ public class RestClientRestIntegrationTests {
 		assertThat(this.client.test()).isEqualTo("2");
 	}
 
+	@Test
+	public void mockServerCallWithContent() throws Exception {
+		this.server.expect(requestTo("/test")).andExpect(content().string("test"))
+				.andRespond(withSuccess("1", MediaType.TEXT_HTML));
+		this.client.testPostWithBody("test");
+	}
+
 }

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

@@ -25,6 +25,7 @@ import org.springframework.boot.web.client.RootUriTemplateHandler;
 import org.springframework.http.client.ClientHttpRequest;
 import org.springframework.http.client.ClientHttpResponse;
 import org.springframework.http.client.support.HttpRequestWrapper;
+import org.springframework.mock.http.client.MockClientHttpRequest;
 import org.springframework.test.web.client.ExpectedCount;
 import org.springframework.test.web.client.MockRestServiceServer;
 import org.springframework.test.web.client.MockRestServiceServer.MockRestServiceServerBuilder;
@@ -77,8 +78,7 @@ public class RootUriRequestExpectationManager implements RequestExpectationManag
 			throws IOException {
 		String uri = request.getURI().toString();
 		if (uri.startsWith(this.rootUri)) {
-			uri = uri.substring(this.rootUri.length());
-			request = new ReplaceUriClientHttpRequest(uri, request);
+			request = replaceURI(request, uri.substring(this.rootUri.length()));
 		}
 		try {
 			return this.expectationManager.validateRequest(request);
@@ -94,6 +94,22 @@ public class RootUriRequestExpectationManager implements RequestExpectationManag
 		}
 	}
 
+	private ClientHttpRequest replaceURI(ClientHttpRequest request,
+			String replacementUri) {
+		URI uri;
+		try {
+			uri = new URI(replacementUri);
+			if (request instanceof MockClientHttpRequest) {
+				((MockClientHttpRequest) request).setURI(uri);
+				return request;
+			}
+			return new ReplaceUriClientHttpRequest(uri, request);
+		}
+		catch (URISyntaxException ex) {
+			throw new IllegalStateException(ex);
+		}
+	}
+
 	@Override
 	public void verify() {
 		this.expectationManager.verify();
@@ -156,14 +172,9 @@ public class RootUriRequestExpectationManager implements RequestExpectationManag
 
 		private final URI uri;
 
-		ReplaceUriClientHttpRequest(String uri, ClientHttpRequest request) {
+		ReplaceUriClientHttpRequest(URI uri, ClientHttpRequest request) {
 			super(request);
-			try {
-				this.uri = new URI(uri);
-			}
-			catch (URISyntaxException ex) {
-				throw new IllegalStateException(ex);
-			}
+			this.uri = uri;
 		}
 
 		@Override