What version of Go are you using (go version)?

$ go version
go version devel +5efe9a8f11 Wed Jan 9 07:21:16 2019 +0000 darwin/amd64

Does this issue reproduce with the latest release?

No, this is an issue for a merged but unreleased change.

What operating system and processor architecture are you using (go env)?

N/A

What did you do?

rproxy := httputil.NewSingleHostReverseProxy(backURL)
rproxy.ModifyResponse = func(resp *http.Response) error {
	resp.Header.Set("X-Important-Header", "HelloWorld")
	return nil
}

req, _ := http.NewRequest("GET", frontendProxy.URL, nil)
req.Header.Set("Connection", "Upgrade")
req.Header.Set("Upgrade", "websocket")

c := frontendProxy.Client()
res, err := c.Do(req)

if res.Header.Get("X-Important-Header") != "HelloWorld" {
	t.Fatalf("missing/invalid custom header; got %#v", res.Header)
}

Full runnable test code: https://gist.github.com/blixt/48f4581437eab9863b977ca7dc3001af

What did you expect to see?

The custom header.

What did you see instead?

--- FAIL: TestReverseProxyWebSocketModifyResponse (0.00s)
    httputil_test.go:67: missing/invalid custom header; got http.Header{"Connection":[]string{"upgrade"}, "Upgrade":[]string{"WebSocket"}}

Remarks

Obviously the ModifyResponse function cannot do anything with Body so the web socket path would ideally clear the Body field in the response, and make sure it's still nil after calling ModifyResponse.