net/http: unset proxy environment after test

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>

src/net/http/proxy_test.go

@@ -75,7 +75,7 @@ func TestCacheKeys(t *testing.T) {
 
 func ResetProxyEnv() {
 	for _, v := range []string{"HTTP_PROXY", "http_proxy", "NO_PROXY", "no_proxy"} {
-		os.Setenv(v, "")
+		os.Unsetenv(v)
 	}
 	ResetCachedEnvironment()
 }

src/net/http/transport_test.go

@@ -2243,6 +2243,7 @@ var proxyFromEnvTests = []proxyFromEnvTest{
 
 func TestProxyFromEnvironment(t *testing.T) {
 	ResetProxyEnv()
+	defer ResetProxyEnv()
 	for _, tt := range proxyFromEnvTests {
 		os.Setenv("HTTP_PROXY", tt.env)
 		os.Setenv("HTTPS_PROXY", tt.httpsenv)