GUACAMOLE-839: Ensure all SSO and related mechanisms set Credentials username.

This is necessary to ensure the ${GUAC_USERNAME} token is properly
substituted in cases where SSO and SSO-like auth methods are used.

extensions/guacamole-auth-json/src/main/java/org/apache/guacamole/auth/json/AuthenticationProviderService.java

@@ -76,6 +76,10 @@ public class AuthenticationProviderService {
         if (userData == null)
             throw new GuacamoleInvalidCredentialsException("Permission denied.", CredentialsInfo.EMPTY);
 
+        // Update credentials with username provided via JSON for sake of
+        // ${GUAC_USERNAME} token
+        credentials.setUsername(userData.getUsername());
+
         // Produce AuthenticatedUser associated with derived UserData
         AuthenticatedUser authenticatedUser = authenticatedUserProvider.get();
         authenticatedUser.init(credentials, userData);