Tests for determinism was not working as intended since io.Copybuffer
uses the io.WriterTo if available.

This exposed that level 0 (no compression) changed output
based on the number of writes and buffers given to the
writer.

Previously, Write would emit a new raw block (BTYPE=00) for
every non-empty call to Write.

This CL fixes it such that a raw block is only emitted upon
the following conditions:
 	* A full window is obtained (every 65535 bytes)
 	* Flush is called
 	* Close is called

Change-Id: I807f866d97e2db7820f11febab30a96266a6cbf1
Reviewed-on: https://go-review.googlesource.com/31174
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>