use Excon :persistent option

Excon now has a `:persistent` option, which is `false` by default.
Fog was already non-persistent by default, however this will change some behavior.

Fog was closing the socket (if it existed) _before_ making the request, then leaving it open.
Now the socket will be closed _after_ a non-persistent request.
The request will also include `Connection: close` to notify the server.

This means that a non-persistent request made _after_ a persistent request
would use the persistent socket (if one exists), then close it.
This is consistent with the new Excon behavior.
If you'd rather continue to call `reset` _before_ non-persistent requests, just let me know.