Schedule fetching refresh token (#164)

Because MieleOAuth.refresh_token is an async function, it does not
actually run unless something awaits the resulting coroutine that it
returns.

custom_components/miele/miele_at_home.py

@@ -1,3 +1,4 @@
+import asyncio
 import functools
 import json
 import logging
@@ -174,7 +175,7 @@ def __init__(self, hass, client_id, client_secret, redirect_uri, cache_path=None
         )
 
         if self.authorized:
-            self.refresh_token(hass)
+            asyncio.create_task(self.refresh_token(hass))
 
     @property
     def authorized(self):