function FormData(options) {
  if (!(this instanceof FormData)) {
    return new FormData();
  }

  this._overheadLength = 0;
  this._valueLength = 0;
  this._valuesToMeasure = [];

  CombinedStream.call(this);

  options = options || {};
  for (var option in options) {
    this[option] = options[option];
  }
}
Adds a default Date header, returning its value.

@return {String}