This is part 2 of 2 for issue 4028.

benchmark                        old ns/op    new ns/op    delta
BenchmarkWriterCopyOptimal           53293        28326  -46.85%
BenchmarkWriterCopyUnoptimal         53757        30537  -43.19%
BenchmarkWriterCopyNoReadFrom        53192        36642  -31.11%

Fixes #4028.

R=nigeltao
CC=golang-dev
https://golang.org/cl/6565056