Make test-button work for multidomain configurations

When validating the configuration, the domain is split at ',' and
each part will be validated individually. However, the actually bind
will use the original domain string with ',' which doesn't work.

src/main/java/hudson/plugins/active_directory/ActiveDirectorySecurityRealm.java

@@ -308,7 +308,7 @@ public FormValidation doValidate(@QueryParameter(fixEmpty = true) String domain,
                             DirContext context = bind(bindName, Secret.toString(password), servers);
                             try {
                                 // actually do a search to make sure the credential is valid
-                                new LDAPSearchBuilder(context, toDC(domain)).searchOne("(objectClass=user)");
+                                new LDAPSearchBuilder(context, toDC(name)).searchOne("(objectClass=user)");
                             } finally {
                                 context.close();
                             }