Add a @EnableWebSecurity if it looks like the user needs one

If the user explicitly disables the basic security features and forgets to
@EnableWebSecurity, and yet still wants a bean of type
WebSecurityConfigurerAdapter, he is trying to use a custom
security setup and the app would fail in a confusing way without
this change.

Fixes gh-568

spring-boot-samples/spring-boot-sample-web-secure/src/test/java/sample/ui/secure/SampleSecureApplicationTests.java

@@ -42,7 +42,7 @@ import static org.junit.Assert.assertTrue;
  * @author Dave Syer
  */
 @RunWith(SpringJUnit4ClassRunner.class)
-@SpringApplicationConfiguration(classes = SampleSecureApplication.class)
+@SpringApplicationConfiguration(classes = SampleWebSecureApplication.class)
 @WebAppConfiguration
 @IntegrationTest
 @DirtiesContext