http: clarify ServeHTTP return
http: clarify ServeHTTP return

Fixes issue 580.

R=adg
CC=golang-dev
http://codereview.appspot.com/207086

tianon/debian-golang,tianon/debian-golang,bdwalton/debian-golang,bdwalton/debian-golang,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,dsriram/golang,nightlyone/golang,h8liu/golang,nightlyone/golang,bdwalton/debian-golang,tianon/debian-golang,tianon/debian-golang,dsriram/golang,apparentlymart/golang,tianon/debian-golang,tianon/debian-golang,nightlyone/golang,bdwalton/debian-golang,h8liu/golang,h8liu/golang,nightlyone/golang,apparentlymart/golang,tianon/debian-golang,dsriram/golang,nightlyone/golang,ssrl/go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,h8liu/golang,pombredanne/go-deleteme,bdwalton/debian-golang,bdwalton/debian-golang,gnanderson/go,h8liu/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,ssrl/go,apparentlymart/golang,tv42/old-go,tv42/old-go,nightlyone/golang,tv42/old-go,bdwalton/debian-golang,tv42/old-go,ssrl/go,pombredanne/go-deleteme,pombredanne/go-deleteme,ssrl/go,h8liu/golang,tianon/debian-golang,gnanderson/go,tv42/old-go,dsriram/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,h8liu/golang,ssrl/go,pombredanne/go-deleteme,ssrl/go,tv42/old-go,bdwalton/debian-golang,apparentlymart/golang,gnanderson/go,nightlyone/golang,pombredanne/go-deleteme