Update connection.py (#1311)

Based on my headaches with silent revert to `localhost` I submit this PR.
The developer should be notified if their host settings are not found.
Details on the issue are here: celery/celery#6661

kombu/connection.py

@@ -627,7 +627,12 @@ def _info(self, resolve=True):
                 transport_cls, transport_cls)
         D = self.transport.default_connection_params
 
-        hostname = self.hostname or D.get('hostname')
+        if not self.hostname:
+            logger.warning(f"No hostname was supplied. Reverting to default '{D.get('hostname')}'")
+            hostname = D.get('hostname')
+        else:
+            hostname = self.hostname
+
         if self.uri_prefix:
             hostname = f'{self.uri_prefix}+{hostname}'