Specify that log file endpoint's response is UTF-8 encoded

Closes gh-16189

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/logging/LogFileWebEndpoint.java

@@ -55,7 +55,7 @@ public class LogFileWebEndpoint {
 		this(environment, null);
 	}
 
-	@ReadOperation(produces = "text/plain")
+	@ReadOperation(produces = "text/plain; charset=UTF-8")
 	public Resource logFile() {
 		Resource logFileResource = getLogFileResource();
 		if (logFileResource == null || !logFileResource.isReadable()) {

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/logging/LogFileWebEndpointWebIntegrationTests.java

@@ -69,6 +69,7 @@ public class LogFileWebEndpointWebIntegrationTests {
 		TestPropertyValues.of("logging.file:" + this.logFile.getAbsolutePath())
 				.applyTo(context);
 		client.get().uri("/actuator/logfile").exchange().expectStatus().isOk()
+				.expectHeader().contentType("text/plain; charset=UTF-8")
 				.expectBody(String.class).isEqualTo("--TEST--");
 	}
 
@@ -77,7 +78,9 @@ public class LogFileWebEndpointWebIntegrationTests {
 		TestPropertyValues.of("logging.file:" + this.logFile.getAbsolutePath())
 				.applyTo(context);
 		client.get().uri("/actuator/logfile").accept(MediaType.TEXT_PLAIN).exchange()
-				.expectStatus().isOk().expectBody(String.class).isEqualTo("--TEST--");
+				.expectStatus().isOk().expectHeader()
+				.contentType("text/plain; charset=UTF-8").expectBody(String.class)
+				.isEqualTo("--TEST--");
 	}
 
 	@Configuration