A connection shouldn't be made available
for re-use until its body has been consumed.

(except in the case of pipelining, which isn't
implemented yet)

This CL fixes some issues seen with heavy load
against Amazon S3.

Subtle implementation detail: to prevent a race
with the client requesting a new connection
before previous one is returned, we actually
have to call putIdleConnection _before_ we
return from the final Read/Close call on the
http.Response.Body.

R=rsc, adg
CC=golang-dev
https://golang.org/cl/4351048