Handle explicit disablement of management SSL correctly

Closes gh-9423

Author: wilkinsona

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

@@ -163,7 +163,7 @@ public void afterSingletonsInstantiated() {
 		}
 		if (managementPort == ManagementServerPort.SAME) {
 			if (new RelaxedPropertyResolver(this.applicationContext.getEnvironment(),
-					"management.ssl.").getProperty("enabled") != null) {
+					"management.ssl.").getProperty("enabled", Boolean.class, false)) {
 				throw new IllegalStateException(
 						"Management-specific SSL cannot be configured as the management "
 								+ "server is not listening on a separate port");

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/autoconfigure/EndpointWebMvcAutoConfigurationTests.java

@@ -558,6 +558,17 @@ public void managementSpecificSslUsingSamePortFails() throws Exception {
 		this.applicationContext.refresh();
 	}
 
+	@Test
+	public void samePortCanBeUsedWhenManagementSslIsExplicitlyDisabled()
+			throws Exception {
+		EnvironmentTestUtils.addEnvironment(this.applicationContext,
+				"management.ssl.enabled=false");
+		this.applicationContext.register(RootConfig.class, EndpointConfig.class,
+				BaseConfiguration.class, EndpointWebMvcAutoConfiguration.class,
+				ErrorMvcAutoConfiguration.class, ServerPortConfig.class);
+		this.applicationContext.refresh();
+	}
+
 	@Test
 	public void managementServerCanDisableSslWhenUsingADifferentPort() throws Exception {
 		EnvironmentTestUtils.addEnvironment(this.applicationContext,