json package: Fixed handling of nil values

Fixes #400.

R=golang-dev, rsc
https://golang.org/cl/167058

src/pkg/json/struct.go

@@ -377,6 +377,11 @@ func writeStruct(w io.Writer, val *reflect.StructValue) os.Error {
 }
 
 func writeValue(w io.Writer, val reflect.Value) (err os.Error) {
+	if val == nil {
+		fmt.Fprint(w, "null");
+		return;
+	}
+
 	switch v := val.(type) {
 	case *reflect.StringValue:
 		fmt.Fprintf(w, "%q", v.Get())
@@ -389,10 +394,15 @@ func writeValue(w io.Writer, val reflect.Value) (err os.Error) {
 	case *reflect.StructValue:
 		err = writeStruct(w, v)
 	case *reflect.ChanValue,
-		*reflect.InterfaceValue,
 		*reflect.PtrValue,
 		*reflect.UnsafePointerValue:
 		err = &MarshalError{val.Type()}
+	case *reflect.InterfaceValue:
+		if v.IsNil() {
+			fmt.Fprint(w, "null")
+		} else {
+			err = &MarshalError{val.Type()}
+		}
 	default:
 		value := val.(reflect.Value);
 		fmt.Fprint(w, value.Interface());

src/pkg/json/struct_test.go

@@ -177,6 +177,7 @@ type marshalTest struct {
 
 var marshalTests = []marshalTest{
 	// basic string
+	marshalTest{nil, "null"},
 	marshalTest{true, "true"},
 	marshalTest{false, "false"},
 	marshalTest{123, "123"},
@@ -185,11 +186,14 @@ var marshalTests = []marshalTest{
 	marshalTest{"teststring", `"teststring"`},
 	marshalTest{[4]int{1, 2, 3, 4}, "[1,2,3,4]"},
 	marshalTest{[]int{1, 2, 3, 4}, "[1,2,3,4]"},
+	marshalTest{[]interface{}{nil}, "[null]"},
 	marshalTest{[][]int{[]int{1, 2}, []int{3, 4}}, "[[1,2],[3,4]]"},
 	marshalTest{map[string]string{"one": "one"}, `{"one":"one"}`},
 	marshalTest{map[string]int{"one": 1}, `{"one":1}`},
+	marshalTest{map[string]interface{}{"null": nil}, `{"null":null}`},
 	marshalTest{struct{}{}, "{}"},
 	marshalTest{struct{ a int }{1}, `{"a":1}`},
+	marshalTest{struct{ a interface{} }{nil}, `{"a":null}`},
 	marshalTest{struct {
 		a	int;
 		b	string;