Fix http status code with unknown log level

This commit changes the http status code to 400 when an unknown log level
is specified.

Closes gh-8798
See gh-8813

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/mvc/LoggersMvcEndpointTests.java

@@ -64,6 +64,7 @@ import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.
  *
  * @author Ben Hale
  * @author Phillip Webb
+ * @author Eddú Meléndez
  */
 @RunWith(SpringRunner.class)
 @SpringBootTest
@@ -169,6 +170,14 @@ public class LoggersMvcEndpointTests {
 		verifyZeroInteractions(this.loggingSystem);
 	}
 
+	@Test
+	public void setLoggerWithWrongLogLevel() throws Exception {
+		this.mvc.perform(post("/loggers/ROOT").contentType(MediaType.APPLICATION_JSON)
+				.content("{\"configuredLevel\":\"other\"}"))
+				.andExpect(status().is4xxClientError());
+		verifyZeroInteractions(this.loggingSystem);
+	}
+
 	@Configuration
 	@Import({ JacksonAutoConfiguration.class,
 			HttpMessageConvertersAutoConfiguration.class,