Fix parsing @context property

Previously this would fail if the @context property also contained
something other than a string, like Mastodon does.

activitystreams/activity.go

@@ -20,7 +20,7 @@ type Activity struct {
 func NewCreateActivity(o *Object) *Activity {
 	a := Activity{
 		BaseObject: BaseObject{
-			Context: []string{
+			Context: []interface{}{
 				"https://www.w3.org/ns/activitystreams",
 			},
 			ID:   o.ID + "/activity",

activitystreams/data.go

@@ -4,7 +4,7 @@ import "fmt"
 
 type (
 	BaseObject struct {
-		Context []string `json:"@context,omitempty"`
+		Context []interface{} `json:"@context,omitempty"`
 		Type    string        `json:"type"`
 		ID      string        `json:"id"`
 	}
@@ -33,7 +33,7 @@ type OrderedCollection struct {
 func NewOrderedCollection(accountRoot, collType string, items int) *OrderedCollection {
 	oc := OrderedCollection{
 		BaseObject: BaseObject{
-			Context: []string{
+			Context: []interface{}{
 				"https://www.w3.org/ns/activitystreams",
 			},
 			ID:   accountRoot + "/" + collType,
@@ -57,7 +57,7 @@ type OrderedCollectionPage struct {
 func NewOrderedCollectionPage(accountRoot, collType string, items, page int) *OrderedCollectionPage {
 	ocp := OrderedCollectionPage{
 		BaseObject: BaseObject{
-			Context: []string{
+			Context: []interface{}{
 				"https://www.w3.org/ns/activitystreams",
 			},
 			ID:   fmt.Sprintf("%s/%s?page=%d", accountRoot, collType, page),

activitystreams/person.go

@@ -18,7 +18,7 @@ func NewPerson(accountRoot string) *Person {
 	p := Person{
 		BaseObject: BaseObject{
 			Type: "Person",
-			Context: []string{
+			Context: []interface{}{
 				"https://www.w3.org/ns/activitystreams",
 			},
 			ID: accountRoot,