GUACAMOLE-96: Add API support for augmenting functionality of other extensions.

extensions/guacamole-auth-header/src/main/java/org/apache/guacamole/auth/header/HTTPHeaderAuthenticationProvider.java

@@ -107,6 +107,13 @@ public class HTTPHeaderAuthenticationProvider implements AuthenticationProvider
 
     }
 
+    @Override
+    public UserContext decorate(UserContext context,
+            AuthenticatedUser authenticatedUser, Credentials credentials)
+            throws GuacamoleException {
+        return context;
+    }
+
     @Override
     public void shutdown() {
         // Do nothing