setRequestHeader = function(value, key) {
      if (this.setRequestHeader) {
        this.setRequestHeader(key, value);
      }
    }
Wrapper around xmlHttp.setRequestHeader to be usable with parameter
order of goog.object.forEach
@this {!XhrLike.OrNative}
@param {string} value The value of the header.
@param {string} key The key of the header;