net/http/httputil: also remove non-standard Proxy-Connection hop-by-hop header

libcurl sends this (despite never being standardized), and the Google
GFE rejects it with a 400 bad request (but only when over http2?).

So nuke it.

Change-Id: I3fc95523d50f33a0e23bb26b9195f70ab0aed0f4
Reviewed-on: https://go-review.googlesource.com/19184Reviewed-by: Chris Broadfoot <cbro@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -106,6 +106,7 @@ func copyHeader(dst, src http.Header) {
 // http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
 var hopHeaders = []string{
 	"Connection",
+	"Proxy-Connection", // non-standard but still sent by libcurl and rejected by e.g. google
 	"Keep-Alive",
 	"Proxy-Authenticate",
 	"Proxy-Authorization",

src/net/http/httputil/reverseproxy_test.go

@@ -45,6 +45,9 @@ func TestReverseProxy(t *testing.T) {
 		if c := r.Header.Get("Upgrade"); c != "" {
 			t.Errorf("handler got Upgrade header value %q", c)
 		}
+		if c := r.Header.Get("Proxy-Connection"); c != "" {
+			t.Errorf("handler got Proxy-Connection header value %q", c)
+		}
 		if g, e := r.Host, "some-name"; g != e {
 			t.Errorf("backend got Host header %q, want %q", g, e)
 		}
@@ -72,6 +75,7 @@ func TestReverseProxy(t *testing.T) {
 	getReq, _ := http.NewRequest("GET", frontend.URL, nil)
 	getReq.Host = "some-name"
 	getReq.Header.Set("Connection", "close")
+	getReq.Header.Set("Proxy-Connection", "should be deleted")
 	getReq.Header.Set("Upgrade", "foo")
 	getReq.Close = true
 	res, err := http.DefaultClient.Do(getReq)