net/http/httputil: fix spelling of Trailer hop-by-hop header per errata

RFC Errata 4522 (http://www.rfc-editor.org/errata_search.php?eid=4522)
notes that RFC 2616 had a typo in a list of headers that the
httputil.ReverseProxy code copied. Fix the typo in our code.

Fixes #14174

Change-Id: Ifc8f18fd58a6508a02a23e54ff3c473f03e521d3
Reviewed-on: https://go-review.googlesource.com/19133Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -109,8 +109,8 @@ var hopHeaders = []string{
 	"Keep-Alive",
 	"Proxy-Authenticate",
 	"Proxy-Authorization",
-	"Te", // canonicalized version of "TE"
-	"Trailers",
+	"Te",      // canonicalized version of "TE"
+	"Trailer", // not Trailers per URL above; http://www.rfc-editor.org/errata_search.php?eid=4522
 	"Transfer-Encoding",
 	"Upgrade",
 }

src/net/http/httputil/reverseproxy_test.go

@@ -48,6 +48,7 @@ func TestReverseProxy(t *testing.T) {
 		if g, e := r.Host, "some-name"; g != e {
 			t.Errorf("backend got Host header %q, want %q", g, e)
 		}
+		w.Header().Set("Trailers", "not a special header field name")
 		w.Header().Set("Trailer", "X-Trailer")
 		w.Header().Set("X-Foo", "bar")
 		w.Header().Set("Upgrade", "foo")
@@ -86,6 +87,9 @@ func TestReverseProxy(t *testing.T) {
 	if c := res.Header.Get(fakeHopHeader); c != "" {
 		t.Errorf("got %s header value %q", fakeHopHeader, c)
 	}
+	if g, e := res.Header.Get("Trailers"), "not a special header field name"; g != e {
+		t.Errorf("header Trailers = %q; want %q", g, e)
+	}
 	if g, e := len(res.Header["X-Multi-Value"]), 2; g != e {
 		t.Errorf("got %d X-Multi-Value header values; expected %d", g, e)
 	}