encoding/xml: Marshal ",any" fields

Fixes #3559.

This makes Marshal handle fields marked ",any" instead of ignoring
them. That makes Marshal more symmetrical with Unmarshal, which seems
to have been a design goal.

Note some test cases were changed, because this patch changes
marshalling behavior. I think the previous behavior was buggy, but
there's still a backward-compatibility question to consider.

R=rsc
CC=golang-dev, n13m3y3r
https://golang.org/cl/6938068

src/pkg/encoding/xml/marshal.go

@@ -273,7 +273,7 @@ func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 	s := parentStack{printer: p}
 	for i := range tinfo.fields {
 		finfo := &tinfo.fields[i]
-		if finfo.flags&(fAttr|fAny) != 0 {
+		if finfo.flags&(fAttr) != 0 {
 			continue
 		}
 		vf := finfo.value(val)
@@ -340,7 +340,7 @@ func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
 				continue
 			}
 
-		case fElement:
+		case fElement, fElement | fAny:
 			s.trim(finfo.parents)
 			if len(finfo.parents) > len(s.stack) {
 				if vf.Kind() != reflect.Ptr && vf.Kind() != reflect.Interface || !vf.IsNil() {