Deserializes a resource's base64 encoded Protobuf header.

def _parse_header(cls, header_proto, resource):
        """Deserializes a resource's base64 encoded Protobuf header.
        """
        header = header_proto()
        try:
            header_bytes = base64.b64decode(resource['header'])
            header.ParseFromString(header_bytes)
        except (KeyError, TypeError, ValueError, DecodeError):
            header = resource.get('header', None)
            LOGGER.error(
                'The validator sent a resource with %s %s',
                'a missing header' if header is None else 'an invalid header:',
                header or '')
            raise errors.ResourceHeaderInvalid()

        resource['header'] = cls._message_to_dict(header)
        return resource