DefaultClient state fix when client is stopped

[bbijelic]

Hi,

consider following:

...
client.stop();
client.isStarted(); // Returns true

After stopping client, I would expect client.isStarted() to return false.

This pull request fixes this issue by simply setting started flag to false in stop() method.

Cheers,
Bojan

[atomashpolskiy]

@bbijelic , thanks !