Refresh credentials to check validity

The original call to credentials.valid does not work until the
credential has been refreshed once, as discussed in #486. This
instead checks if the credential will work by trying to connect

gcsfs/credentials.py

@@ -92,9 +92,12 @@ def _connect_google_default(self):
 
     def _connect_cloud(self):
         self.credentials = gauth.compute_engine.Credentials()
-
-        if not self.credentials.valid:
-            raise ValueError("Invalid gcloud credentials")
+        try:
+            with requests.Session() as session:
+                req = Request(session)
+                self.credentials.refresh(req)
+        except gauth.exceptions.RefreshError as error:
+            raise ValueError("Invalid gcloud credentials") from error
 
     def _connect_cache(self):