code: function (headers, separator) {
          if (!_.isArray(headers) && !PropertyList.isPropertyList(headers)) {
              return E;
          }
  
          return headers.map(Header.unparseSingle).join(separator ? separator : '\n');
      }
docs: Stringifies an Array or {@link PropertyList} of Headers into a single string.
  
  @param {Array|PropertyList<Header>} headers
  @param {String=} separator - Specify a string for separating each header, by default, '\n', but sometimes,
  it might be more useful to use a carriage return ('\r\n')
  @returns {string}