Prevents a ton of garbage. (Noticed this when writing large
Camlistore zip archives to Amazon Glacier)

Note that the Closer part of the io.WriteCloser is never given
to users. It's an internal detail of the package.

benchmark                         old ns/op     new ns/op     delta
BenchmarkCompressedZipGarbage     42884123      40732373      -5.02%

benchmark                         old allocs     new allocs     delta
BenchmarkCompressedZipGarbage     204            149            -26.96%

benchmark                         old bytes     new bytes     delta
BenchmarkCompressedZipGarbage     4397576       66744         -98.48%

LGTM=adg, rsc
R=adg, rsc
CC=golang-codereviews
https://golang.org/cl/54300053