Change default client to session

Author: Joohwan Oh

arango/api.py

@@ -35,7 +35,7 @@ def __init__(self, protocol="http", host="localhost", port=8529,
         self.password = password
         self.db_name = db_name
         # self.client = SessionArangoClient() if client is None else client
-        self.client = DefaultArangoClient() if client is None else client
+        self.client = SessionArangoClient() if client is None else client
 
     @property
     def url_prefix(self):