Fixing some tests

tests/mocks/object_with_tags.json

@@ -6,7 +6,8 @@
   "tag": [
     {
       "name": "#my_tag",
-      "id":  "http://example.com/tag/my_tag"
+      "id":  "http://example.com/tag/my_tag",
+      "type": "Mention"
     },
     {
       "name": "@ana",

tests/unmarshalling_test.go

@@ -241,11 +241,12 @@ var allTests = tests{
 					a.NilLangRef, "A Simple, non-specific object",
 				}},
 				Tag: a.ItemCollection{
-					&a.Object{
+					&a.Mention{
 						Name: a.NaturalLanguageValues{{
 							a.NilLangRef, "#my_tag",
 						}},
 						ID: a.ObjectID("http://example.com/tag/my_tag"),
+						Type: a.MentionType,
 					},
 					&a.Mention{
 						Name: a.NaturalLanguageValues{{
@@ -512,3 +513,8 @@ func Test_ActivityPubUnmarshal(t *testing.T) {
 		})
 	}
 }
+
+func TestMain (m *testing.M) {
+	a.ItemTyperFunc = ap.JSONGetItemByType
+	m.Run()
+}