Completely update member references from message inner members.

Fixes #5819

discord/member.py

@@ -199,6 +199,12 @@ class Member(discord.abc.Messageable, _BaseUser):
         data['user'] = author._to_minimal_user_json()
         return cls(data=data, guild=message.guild, state=message._state)
 
+    def _update_from_message(self, data):
+        self.joined_at = utils.parse_time(data.get('joined_at'))
+        self.premium_since = utils.parse_time(data.get('premium_since'))
+        self._update_roles(data)
+        self.nick = data.get('nick', None)
+
     @classmethod
     def _try_upgrade(cls, *,  data, guild, state):
         # A User object with a 'member' key

discord/message.py

@@ -476,8 +476,7 @@ class Message:
         author = self.author
         try:
             # Update member reference
-            if author.joined_at is None:
-                author.joined_at = utils.parse_time(member.get('joined_at'))
+            author._update_from_message(member)
         except AttributeError:
             # It's a user here
             # TODO: consider adding to cache here