code: function parseHeaders(filename, headerLines, only = null) {
    const headers = Object.create(null);
    for (let line of headerLines) {
      if (line === '')
        continue;
      let [name, value] = line.match(/^(.*?)\:\s+(.*)$/).slice(1);
      if (only && !match(name, only))
        continue;
  
      const key  = (name || '').toLowerCase();
      value = (value || '').trim().replace(/^"(.*)"$/, '$1');
      if (Array.isArray(headers[key]))
        headers[key].push(value);
      else if (headers[key])
        headers[key] = [headers[key], value];
      else
        headers[key] = value;
    }
    return headers;
  }
docs: Parse headers from headerLines.  Optional argument `only` is an array of regular expressions; only headers matching one of these expressions are parsed.  Returns a object with name/value pairs.