code: function(realm, err, cbreqauth) {
          if (!(err instanceof Exc.jsDAV_Exception))
              err = new Exc.NotAuthenticated(err);
  
          var currQop = "";
          switch (this.qop) {
              case QOP_AUTH:
                  currQop = "auth";
                  break;
              case QOP_AUTHINT:
                  currQop = "auth-int";
                  break;
              case QOP_AUTH | QOP_AUTHINT:
                  currQop = "auth,auth-int";
                  break;
          }
  
          err.addHeader("WWW-Authenticate", "Digest realm=\"" + realm + 
              "\",qop=\"" + currQop + "\",nonce=\"" + this.nonce + 
              "\",opaque=\"" + this.opaque + "\"");
          cbreqauth(err, false);
      }
docs: Returns an HTTP 401 header, forcing login
  
  This should be called when username and password are incorrect, or not supplied at all
  
  @return void