Fix #5354 & #5355 - Fix Graveyard.serialize

Graveyard.serialize raised an exception as the deleted datetime
objects where not being converted to strings for json encoding. It
also didn't deal with the case when there was an actor, it now calls
the actor's serialize method as it should.

Thanks to Alyeph who provided the patches for both of these.

mediagoblin/db/models.py

@@ -389,11 +389,14 @@ class LocalUser(User):
                 'admin' if self.has_privilege(u'admin') else 'user',
                 self.username)
 
+    def get_public_id(self, host):
+        return "acct:{0}@{1}".format(self.username, host)
+
     def serialize(self, request):
         user = {
-            "id": "acct:{0}@{1}".format(self.username, request.host),
+            "id": self.get_public_id(request.host),
             "preferredUsername": self.username,
-            "displayName": "{0}@{1}".format(self.username, request.host),
+            "displayName": self.get_public_id(request.host).split(":", 1)[1],
             "links": {
                 "self": {
                     "href": request.urlgen(
@@ -1563,15 +1566,19 @@ class Graveyard(Base):
         )
 
     def serialize(self, request):
-        return {
+        deleted = UTC.localize(self.deleted).isoformat()
+        context = {
             "id": self.public_id,
             "objectType": self.object_type,
-            "actor": self.actor(),
-            "published": self.deleted,
-            "updated": self.deleted,
-            "deleted": self.deleted
+            "published": deleted,
+            "updated": deleted,
+            "deleted": deleted,
         }
 
+        if self.actor_id is not None:
+            context["actor"] = self.actor().serialize(request)
+
+        return context
 MODELS = [
     LocalUser, RemoteUser, User, MediaEntry, Tag, MediaTag, Comment, TextComment,
     Collection, CollectionItem, MediaFile, FileKeynames, MediaAttachmentFile,