Fix JSP EL support with Tomcat

Fix TomcatEmbeddedServletContainerFactory to set a MERGED_WEB_XML
attribute when JSPs are used. This is required for EL support with
JSPs since Jasper checks the version number in the web.xml. Without
any web.xml Jasper default to disabling EL.

Issue: #55752948

spring-boot-samples/spring-boot-sample-web-jsp/src/test/java/org/springframework/boot/sample/jsp/SampleWebJspApplicationTests.java

@@ -0,0 +1,72 @@
+package org.springframework.boot.sample.jsp;
+
+import java.io.IOException;
+import java.util.concurrent.Callable;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.springframework.boot.SpringApplication;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.http.client.ClientHttpResponse;
+import org.springframework.web.client.DefaultResponseErrorHandler;
+import org.springframework.web.client.RestTemplate;
+
+import static org.junit.Assert.*;
+
+/**
+ * Basic integration tests for JSP application.
+ *
+ * @author Phillip Webb
+ */
+public class SampleWebJspApplicationTests {
+
+	private static ConfigurableApplicationContext context;
+
+	@BeforeClass
+	public static void start() throws Exception {
+		Future<ConfigurableApplicationContext> future = Executors
+				.newSingleThreadExecutor().submit(
+						new Callable<ConfigurableApplicationContext>() {
+							@Override
+							public ConfigurableApplicationContext call() throws Exception {
+								return (ConfigurableApplicationContext) SpringApplication
+										.run(SampleWebJspApplication.class);
+							}
+						});
+		context = future.get(60, TimeUnit.SECONDS);
+	}
+
+	@AfterClass
+	public static void stop() {
+		if (context != null) {
+			context.close();
+		}
+	}
+
+	@Test
+	public void testJspWithEl() throws Exception {
+		ResponseEntity<String> entity = getRestTemplate().getForEntity(
+				"http://localhost:8080", String.class);
+		assertEquals(HttpStatus.OK, entity.getStatusCode());
+		assertTrue("Wrong body:\n" + entity.getBody(), entity
+				.getBody().contains("/resources/text.txt"));
+	}
+
+	private RestTemplate getRestTemplate() {
+		RestTemplate restTemplate = new RestTemplate();
+		restTemplate.setErrorHandler(new DefaultResponseErrorHandler() {
+			@Override
+			public void handleError(ClientHttpResponse response) throws IOException {
+			}
+		});
+		return restTemplate;
+
+	}
+
+}