net/http: on Transport body write error, wait briefly for a response

From https://github.com/golang/go/issues/11745#issuecomment-123555313 :

The http.RoundTripper interface says you get either a *Response, or an
error.

But in the case of a client writing a large request and the server
replying prematurely (e.g. 403 Forbidden) and closing the connection
without reading the request body, what does the client want? The 403
response, or the error that the body couldn't be copied?

This CL implements the aforementioned comment's option c), making the
Transport give an N millisecond advantage to responses over body write
errors.

Updates #11745

Change-Id: I4485a782505d54de6189f6856a7a1f33ce4d5e5e
Reviewed-on: https://go-review.googlesource.com/12590


Reviewed-by: Russ Cox <rsc@golang.org>