net/http: change test to use override param instead of chan.
net/http: change test to use override param instead of chan.

Follow-on from http://codereview.appspot.com/5543062.

R=bradfitz, dvyukov
CC=golang-dev
http://codereview.appspot.com/5539071

gnanderson/go,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,tv42/old-go,gnanderson/go,gnanderson/go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,tv42/old-go,tv42/old-go