Change default close code to 4000 instead of 1000.

This prevents our sessions from being invalidated by Discord which
caused a lot of failures when resuming. This caused an unnecessary
amount of IDENTIFYs to happen depending on the circumstances.

Closes #2631

See also discord/discord-api-docs#1472

discord/client.py

@@ -549,7 +549,7 @@ class Client:
                 pass
 
         if self.ws is not None and self.ws.open:
-            await self.ws.close()
+            await self.ws.close(code=1000)
 
         self._ready.clear()