Harmonize property name

The property default format is lower case using hyphen. The JMX default
domain property has been harmonized to that format.

Fixes gh-2427

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/autoconfigure/EndpointMBeanExportProperties.java

@@ -30,7 +30,7 @@ import org.springframework.util.StringUtils;
 @ConfigurationProperties(prefix = "endpoints.jmx")
 public class EndpointMBeanExportProperties {
 
-	@Value("${spring.jmx.default_domain:}")
+	@Value("${spring.jmx.default-domain:}")
 	private String domain;
 
 	private boolean uniqueNames = false;

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/integration/IntegrationAutoConfiguration.java

@@ -52,7 +52,7 @@ public class IntegrationAutoConfiguration {
 	@ConditionalOnClass(EnableIntegrationMBeanExport.class)
 	@ConditionalOnMissingBean(value = IntegrationMBeanExporter.class, search = SearchStrategy.CURRENT)
 	@ConditionalOnExpression("${spring.jmx.enabled:true}")
-	@EnableIntegrationMBeanExport(defaultDomain = "${spring.jmx.default_domain:}", server = "${spring.jmx.server:mbeanServer}")
+	@EnableIntegrationMBeanExport(defaultDomain = "${spring.jmx.default-domain:}", server = "${spring.jmx.server:mbeanServer}")
 	protected static class IntegrationJmxConfiguration {
 	}
 

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/jmx/JmxAutoConfigurationTests.java

@@ -92,7 +92,7 @@ public class JmxAutoConfigurationTests {
 	public void testDefaultDomainConfiguredOnMBeanExport() {
 		MockEnvironment env = new MockEnvironment();
 		env.setProperty("spring.jmx.enabled", "true");
-		env.setProperty("spring.jmx.default_domain", "my-test-domain");
+		env.setProperty("spring.jmx.default-domain", "my-test-domain");
 		this.context = new AnnotationConfigApplicationContext();
 		this.context.setEnvironment(env);
 		this.context.register(TestConfiguration.class, JmxAutoConfiguration.class);