bufio: make Writer.ReadFrom not flush prematurely. For example,
many small writes to a network may be less efficient that a few
large writes.

This fixes net/http's TestClientWrites, broken by 6565056 that
introduced Writer.ReadFrom. That test implicitly assumed that
calling io.Copy on a *bufio.Writer wouldn't write to the
underlying network until the buffer was full.

R=dsymonds
CC=bradfitz, golang-dev, mchaten, mikioh.mikioh
https://golang.org/cl/6743044