Automatic merge from submit-queue

Unset https_proxy before roundtripper_test

When running `hack/test-go.sh`, if the testing env is behind a https proxy, roundtripper_test will fail randomly.

After `unset https_proxy`, the testing works well. So, add a comment to be a troubleshooting tip.

Fail info:

```
--- FAIL: TestRoundTripAndNewConnection (0.12s)
	roundtripper_test.go:319: proxied http->http: shouldError=false, got true: Get http://127.0.0.1:46711: unexpected EOF
FAIL
FAIL	k8s.io/kubernetes/pkg/util/httpstream/spdy	0.148s
```

```
--- FAIL: TestRoundTripAndNewConnection (0.12s)
	roundtripper_test.go:319: proxied https with auth (valid hostname + RootCAs) -> http: shouldError=false, got true: Get http://127.0.0.1:41028: unexpected EOF
FAIL
FAIL	k8s.io/kubernetes/pkg/util/httpstream/spdy	0.146s
```