If the request included authentication credentials, then the 401 response indicates that authorization has been refused for those credentials. The user agent MAY repeat the request with a new or replaced Authorization header field2. If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user agent SHOULD present the enclosed representation to the user, since it usually contains relevant diagnostic information.
Since Fuel doesn't use basic authentication, it doesn't have any provisions to send WWW-Authenticate headers back.
If you want to use basic or digest authentication, you can use the set_header() method of the response object to set response headers and use the set_status() method to set the HTTP status. Do not use the object property directly if a setter is available, it might not be public in future releases.
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