Allow group feeds to be subscribed to

`IO_MQTT#subscribe` always checks `feed_key` even though it is not required. When `group_key` is provided instead of `feed_key`, `feed_key` should not be required to be valid.

This change only validates `feed_key` when one is provided and adds validation for `group_key` when it is provided.

Author: ryanplusplus

adafruit_io/adafruit_io.py

@@ -253,12 +253,14 @@ def subscribe(self, feed_key=None, group_key=None, shared_user=None):
 
             client.subscribe([('temperature'), ('humidity')])
         """
-        validate_feed_key(feed_key)
         if shared_user is not None and feed_key is not None:
+            validate_feed_key(feed_key)
             self._client.subscribe("{0}/f/{1}".format(shared_user, feed_key))
         elif group_key is not None:
+            validate_feed_key(group_key)
             self._client.subscribe("{0}/g/{1}".format(self._user, group_key))
         elif feed_key is not None:
+            validate_feed_key(feed_key)
             self._client.subscribe("{0}/f/{1}".format(self._user, feed_key))
         else:
             raise AdafruitIO_MQTTError("Must provide a feed_key or group_key.")