Writes gzip-format uncompressed files.

Change-Id: Id5dc01852e21732978700fd9efcc2d4a5899ffeb
Reviewed-on: https://go-review.googlesource.com/32260
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Michael Matloob <matloob@golang.org>