reflect: document and test TypeOf(nil)

Fixes #3549.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6107047

src/pkg/reflect/all_test.go

@@ -638,6 +638,7 @@ var (
 
 var deepEqualTests = []DeepEqualTest{
 	// Equalities
+	{nil, nil, true},
 	{1, 1, true},
 	{int32(1), int32(1), true},
 	{0.5, 0.5, true},
@@ -696,6 +697,10 @@ func TestDeepEqual(t *testing.T) {
 }
 
 func TestTypeOf(t *testing.T) {
+	// Special case for nil
+	if typ := TypeOf(nil); typ != nil {
+		t.Errorf("expected nil type for nil value; got %v", typ)
+	}
 	for _, test := range deepEqualTests {
 		v := ValueOf(test.a)
 		if !v.IsValid() {