PYTHON-1617 Fix database name check in index cache (mongodb#363)

Author: didibz

pymongo/mongo_client.py

@@ -636,7 +636,7 @@ def _cache_index(self, dbname, collection, index, cache_for):
  expire = datetime.timedelta(seconds=cache_for) + now
 
  with self.__index_cache_lock:
- if database not in self.__index_cache:
+ if dbname not in self.__index_cache:
  self.__index_cache[dbname] = {}
  self.__index_cache[dbname][collection] = {}
  self.__index_cache[dbname][collection][index] = expire