net/http: make client await response concurrently with writing request

net/http: make client await response concurrently with writing request

If the server replies with an HTTP response before we're done
writing our body (for instance "401 Unauthorized" response), we
were previously ignoring that, since we returned our write
error ("broken pipe", etc) before ever reading the response.
Now we read and write at the same time.

Fixes #3595

R=rsc, adg
CC=golang-dev
https://golang.org/cl/6238043