set_proposed_credential
Description:
Set the Credential of the proposed authentication challenge.
Set the Credential of the proposed authentication challenge that was stored 
      from a previous session. This should only be used by applications handling their own credential storage. (When using the default WebKit 
      credential storage, get_proposed_credential
       already contains previously-stored credentials.) Passing a null credential will clear the 
      proposed credential.
Parameters:
| this | |
| credential | a Credential, or null |