net/http: return nil from Header.Clone if the receiver is nil

Fixes #33141

Change-Id: I84a8b3496fc9396fd1c09ba9505697c34bdf7105
Reviewed-on: https://go-review.googlesource.com/c/go/+/188022
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/net/http/header.go

@@ -78,8 +78,12 @@ func (h Header) write(w io.Writer, trace *httptrace.ClientTrace) error {
 	return h.writeSubset(w, nil, trace)
 }
 
-// Clone returns a copy of h.
+// Clone returns a copy of h or nil if h is nil.
 func (h Header) Clone() Header {
+	if h == nil {
+		return nil
+	}
+
 	// Find total number of values.
 	nv := 0
 	for _, vv := range h {

src/net/http/header_test.go

@@ -176,6 +176,14 @@ func TestHasToken(t *testing.T) {
 	}
 }
 
+func TestNilHeaderClone(t *testing.T) {
+	t1 := Header(nil)
+	t2 := t1.Clone()
+	if t2 != nil {
+		t.Errorf("cloned header does not match original: got: %+v; want: %+v", t2, nil)
+	}
+}
+
 var testHeader = Header{
 	"Content-Length": {"123"},
 	"Content-Type":   {"text/plain"},