code: function (auth, request, done) {
          var header,
              params = auth.get(AUTH_PARAMETERS),
              url = urlEncoder.toNodeUrl(request.url.toString(true));
  
          if (!params.username || !params.realm) {
              return done(); // Nothing to do if required parameters are not present.
          }
  
          request.removeHeader(AUTHORIZATION, {ignoreCase: true});
  
          params.method = request.method;
          params.uri = url.path;
          params.body = request.body && request.body.toString();
  
          try {
              header = this.computeHeader(params);
          }
          catch (e) { return done(e); }
  
          request.addHeader({
              key: AUTHORIZATION,
              value: header,
              system: true
          });
  
          return done();
      }
docs: Signs a request.
  
  @param {AuthInterface} auth
  @param {Request} request
  @param {AuthHandlerInterface~authSignHookCallback} done