Let utils.get_peer handle DialogPeer

This is important since some updates have a peer of
that type instead of just the normal Peer instance.

telethon/utils.py

@@ -771,6 +771,8 @@ def get_peer(peer):
             return peer.peer
         elif isinstance(peer, types.ChannelFull):
             return types.PeerChannel(peer.id)
+        elif isinstance(peer, types.DialogPeer):
+            return peer.peer
 
         if peer.SUBCLASS_OF_ID in (0x7d7c6f86, 0xd9c7fc18):
             # ChatParticipant, ChannelParticipant