code: function(headers, params) {
          var newHeaders = {};
          for (var header in headers)
              newHeaders[header] = headers[header];
          for (var param in params) {
              var re = new RegExp(":"+ param);
              for (var header in newHeaders) {
                  if (newHeaders[header].search(re) != -1) {
                      newHeaders[header] = newHeaders[header].replace(re, params[param]);
                  }
              }
          }
          return newHeaders;
      }
docs: format final headers