Fix voice state update issue in on_voice_state_update

Bug was caused to the shallow copy not copying over the VoiceState
information embedded into the copy. This would mean that when the event
is called, before and after voice state information is essentially
equivalent.

The solution to fix this is to also copy the VoiceState objects.

discord/state.py

@@ -268,7 +268,7 @@ class ConnectionState:
             member = self._make_member(server, data)
             server._add_member(member)
 
-        old_member = copy.copy(member)
+        old_member = member._copy()
         member.status = data.get('status')
         try:
             member.status = Status(member.status)
@@ -385,7 +385,7 @@ class ConnectionState:
         member = server.get_member(user_id)
         if member is not None:
             user = data['user']
-            old_member = copy.copy(member)
+            old_member = member._copy()
             member.name = user['username']
             member.discriminator = user['discriminator']
             member.avatar = user['avatar']