Fix typo in test comment

Fix typo in test comment, which was referring to an async client
as a sync client.
(TK-27) Allow clients to persist

Modify the handling of clients such that a client that is
explicitly created by a user will not be closed after receiving
a response.
(TK-27) Add test for persistent async client

Add test for persistent async client. This test creates a
persistent async client, then uses it to make each kind of HTTP
request.