It is valid for io.Reader to return (n, io.EOF) where n is positive.
The unit test should not fail if io.EOF is returned when read until
the end.

Change-Id: I7b918e3cc03db8b90c8aa58f4c0f7806a1d4af7e
Reviewed-on: https://go-review.googlesource.com/21307Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>