pushHeader() {
    if (this.opts.withBOM) {
      this.onData('\ufeff');
    }
  
    if (this.opts.header) {
      const header = this.getHeader(this.opts.fields);
      this.onHeader(header);
      this.onData(header);
      this._hasWritten = true;
    }
  }
Generate the csv header and pushes it downstream.