`(net http-client)`'s response may return port, instead of bytevector

[Takashi Kato]

When the server returns nobody response, such as 3xx, then HTTP:response-body returns a port instead of a bytevector.

[Takashi Kato]

Set empty bytevector as a initial response body (Fixes #289)

→ <<cset f40ca8830d7f>>