Fix the socket policy to fix a flaky test.

postFailsWithChunkedRequestForSmallRequest was flaky because the socket policy
wasn't triggering the expected exception at the expected time. Changing to
a different socket policy should fix flakiness.

https://github.com/square/okhttp/issues/651
Merge pull request #708 from square/jwilson_0417_change_socket_policy

Fix the socket policy to fix a flaky test.