http: consume request bodies before replying

This fixes our http behavior (even if Handlers forget to
consume a request body, we do it for them before we send
their response header), fixes the racy TestServerExpect,
and adds TestServerConsumesRequestBody.

With GOMAXPROCS>1, the http tests now seem race-free.

R=rsc
CC=golang-dev
https://golang.org/cl/4419042
2 files changed