Fix last proxy in TestProxyFromEnvironment bleeds into other tests
Change ResetProxyEnv to use the newer os.Unsetenv, instead of hard
coding as ""

Change-Id: I67cf833dbcf4bec2e10ea73c354334160cf05f84
Reviewed-on: https://go-review.googlesource.com/38115
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>