SEC-933: global-method-security and aop:aspectj-autoproxy throws NullPointerException in some situations

http://jira.springframework.org/browse/SEC-933. Removed the setting of the attributeSource field from the interceptor in MethodDefinitionSourceAdvisor as this was overwriting the version supplied with the constructor with null (causing the NPE).
Also implemented lazy initialization of the authentication provider list from the bean factory in a custom NamespaceAuthenticationManager (extends ProviderManager and introspects the BeanFactory when getProviders() is first called). This should prevent the perennial problem of the eager initialization of UserDetailsService and other beans when the interceptor is eagerly initialized by something like aspectj-autoproxy.

core/src/test/java/org/springframework/security/providers/ProviderManagerTests.java

@@ -119,10 +119,11 @@ public class ProviderManagerTests {
     }
 
     @Test(expected=IllegalArgumentException.class)
-    public void startupFailsIfProviderListNotSet() throws Exception {
+    public void getProvidersFailsIfProviderListNotSet() throws Exception {
         ProviderManager mgr = new ProviderManager();
 
         mgr.afterPropertiesSet();
+        mgr.getProviders();
     }
 
     @Test(expected=IllegalArgumentException.class)