Use a String when referencing a class with @ConditionalOnMissingClass

Using a Class reference can cause reflection problems at runtime (see
gh-1065).

Closes gh-2674

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

@@ -158,7 +158,7 @@ public class EndpointWebMvcChildContextConfiguration {
 	 * configures the security filter.
 	 */
 	@Configuration
-	@ConditionalOnMissingClass(WebSecurityConfigurerAdapter.class)
+	@ConditionalOnMissingClass(name = "org.springframework.security.config.annotation.web.configuration.WebSecurityConfigurerAdapter")
 	protected static class EndpointHandlerMappingConfiguration {
 
 		@Autowired(required = false)