net/http: refactor body logic in test
net/http: refactor body logic in test

This just eliminates some duplication.
Also add a pointer to RFC 1122, in case
this comes up again.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/6229044

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