[sentinel] update master address if it changes. (#847)

Update the cached master_address if the Sentinels promote a new master.

Author: jhgg

redis/sentinel.py

@@ -103,6 +103,7 @@ def get_master_address(self):
                 self.master_address = master_address
             elif master_address != self.master_address:
                 # Master address changed, disconnect all clients in this pool
+                self.master_address = master_address
                 self.disconnect()
         return master_address