Return a HeaderSet excluding the headers in the exclude list.

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