Talk about client half-close

[mnot]

Fixes #22.

[irvingreid]

Historical note: HTTP 1.0 arguably specified the "abort request if the client half-closes" behaviour, see the last paragraph of https://www.rfc-editor.org/rfc/rfc1945#section-1.3, two pages down, just above https://www.rfc-editor.org/rfc/rfc1945#section-1.4. This behaviour was implemented by several widely used HTTP servers; I worked on a firewall product at the time, and we had to take great care to make sure we handled TCP half-close correctly...

The "arguably' is that RFC 1945 says "the closing of the connection by either or both parties", but sending a FIN doesn't really "close" the connection - that requires either a RST or both ends ACK-ing the other end's FIN.