json: do not Marshal unexported struct fields

R=r, cw, niemeyer, rsc
CC=golang-dev
https://golang.org/cl/3952041

src/pkg/json/decode_test.go

@@ -270,6 +270,8 @@ type All struct {
 
 	Interface  interface{}
 	PInterface *interface{}
+
+	unexported int
 }
 
 type Small struct {

src/pkg/json/encode.go

@@ -37,6 +37,7 @@ import (
 // a member of the object.  By default the object's key name is the
 // struct field name converted to lower case.  If the struct field
 // has a tag, that tag will be used as the name instead.
+// Only exported fields will be encoded.
 //
 // Map values encode as JSON objects.
 // The map's key type must be string; the object keys are used directly
@@ -219,11 +220,17 @@ func (e *encodeState) reflectValue(v reflect.Value) {
 		e.WriteByte('{')
 		t := v.Type().(*reflect.StructType)
 		n := v.NumField()
+		first := true
 		for i := 0; i < n; i++ {
-			if i > 0 {
+			f := t.Field(i)
+			if f.PkgPath != "" {
+				continue
+			}
+			if first {
+				first = false
+			} else {
 				e.WriteByte(',')
 			}
-			f := t.Field(i)
 			if f.Tag != "" {
 				e.string(f.Tag)
 			} else {