fix: ordering in limit_to_last (#692)

When limit_to_last was set, we need to reverse the order. However due to error in comparing the order direction, it was not properly set.

comparing `order.direction == self.ASCENDING` is always `False` because there are two different types.

The correct way is by comparing `order.direction.name == self.ASCENDING`

Fixes #536

google/cloud/firestore_v1/query.py

@@ -160,7 +160,7 @@ def get(
             for order in self._orders:
                 order.direction = _enum_from_direction(
                     self.DESCENDING
-                    if order.direction == self.ASCENDING
+                    if order.direction.name == self.ASCENDING
                     else self.ASCENDING
                 )
             self._limit_to_last = False