When delivering activities, sign the Digest header

Looks like I forgot to add it to the list of headers to sign

src/Vervis/Foundation.hs

@@ -654,7 +654,7 @@ instance YesodRemoteActorStore App where
 
 instance YesodActivityPub App where
     sitePostSignedHeaders _ =
-        hRequestTarget :| [hHost, hDate, hActivityPubActor]
+        hRequestTarget :| [hHost, hDate, hDigest, hActivityPubActor]
     siteGetHttpSign = do
         (akey1, akey2, new1) <- liftIO . readTVarIO =<< asksSite appActorKeys
         renderUrl <- askUrlRender