docs: def filter(self, exclude):
          """Return a HeaderSet excluding the headers in the exclude list."""
          filtered_headers = HeaderDict()
          lowercased_ignore_list = [x.lower() for x in exclude]
  
          for header, value in iteritems(self):
              if header.lower() not in lowercased_ignore_list:
                  filtered_headers[header] = value
  
          return filtered_headers