Same interface as Net::HTTP but it will never try multiple reads
when the read_response_headers() or read_entity_body() methods are
invoked. This make it possible to multiplex multiple Net::HTTP::NB using select without risk blocking.
If read_response_headers() did not see enough data to complete the
headers an empty list is returned.
If read_entity_body() did not see new entity data in its read
the value -1 is returned.