Update ZabbixAPI.cs

use Query-Object without Param "auth" for API-Calls with method apiinfo.version" || "user.login"

see -> Zabbix Documentation 2.0:
apiinfo.version
string apiinfo.version(array)
 
This method is available to unauthenticated users and should be called without the auth parameter in the JSON-RPC request. Starting from Zabbix 2.4 the method will return an error if the auth parameter is given.