code: function _setHeaders(o, h) {
          var p;
              h = h || {};
  
          for (p in _headers) {
              if (_headers.hasOwnProperty(p)) {
                  if (h[p]) {
                      // Configuration headers will supersede io preset headers,
                      // if headers match.
                      continue;
                  }
                  else {
                      h[p] = _headers[p];
                  }
              }
          }
  
          for (p in h) {
              if (h.hasOwnProperty(p)) {
                  o.setRequestHeader(p, h[p]);
              }
          }
      }
docs: @description Method that sets all HTTP headers to be sent in a transaction.
  
  @method _setHeaders
  @private
  @static
  @param {object} o - XHR instance for the specific transaction.
  @param {object} h - HTTP headers for the specific transaction, as defined
  in the configuration object passed to YUI.io().
  @return void