Add Activity.Updated field

Per the Mastodon docs, we need to send an `updated` parameter for
`Update` activities to work correctly.
https://docs.joinmastodon.org/spec/activitypub/#supported-activities-for-statuses

activitystreams/activity.go

@@ -19,6 +19,7 @@ type Activity struct {
 	BaseObject
 	Actor     string    `json:"actor"`
 	Published time.Time `json:"published,omitempty"`
+	Updated   time.Time `json:"updated,omitempty"`
 	To        []string  `json:"to,omitempty"`
 	CC        []string  `json:"cc,omitempty"`
 	Object    *Object   `json:"object"`
@@ -67,6 +68,7 @@ func NewUpdateActivity(o *Object) *Activity {
 		Actor:     o.AttributedTo,
 		Object:    o,
 		Published: o.Published,
+		Updated:   o.Updated,
 	}
 	return &a
 }
@@ -107,6 +109,7 @@ func NewFollowActivity(actorIRI, followeeIRI string) *FollowActivity {
 type Object struct {
 	BaseObject
 	Published    time.Time         `json:"published,omitempty"`
+	Updated      time.Time         `json:"updated,omitempty"`
 	Summary      *string           `json:"summary,omitempty"`
 	InReplyTo    *string           `json:"inReplyTo,omitempty"`
 	URL          string            `json:"url"`