AuthCallback
Description:
Perform the authentication against the credentials source.
This will typically retreive the password for VSGI.Authorization.username and call VSGI.Authorization.challenge_with_password.
Parameters:
| auth | represent the credentials provided by the user-agent which can be challenged | 
Returns:
| 'true' if the authentication is successful, 'false' otherwise even if the provided input is malformed | 
Namespace: Valum
  
  Package: valum-0.3