/agents/:agent_id/upgrade returns output not in JSON

[jesuslinares]

The agent is upgraded, but the API shows an error.

# curl -u foo:bar -X PUT -k "http://127.0.0.1:55000/agents/005/upgrade?pretty"
{
   "error": 2,
   "message": "Command output not in JSON"
}

# curl -u foo:bar -X GET -k "http://127.0.0.1:55000/agents/005/upgrade_result?pretty"
{
   "error": 0,
   "data": "Agent upgraded successfully"
}