Allowing Message Content Type customisation

Allowing Message Content Type Customisation so custom JSON layouts can
be sent.
Fixes https://github.com/haf/NLog.RabbitMQ/issues/9
```
 <variable name="JSONLayout" value='{"@timestamp":"${date:format=yyyy-MM-ddTHH\:mm\:ssZ}","level":"${level}","@source":"nlog://TODO","@message":"${message:jsonEncode=true}","@fields":{"thread":"${threadid}","logger":"${logger}","exception":"${exception:format=tostring:jsonEncode=true}"}}' />


  <target name="RabbitMQTarget"
                xsi:type="RabbitMQ"
                exchange="app-logging"
                durable="true"
                appid="id"
               contentType="application/json"
                useJSON="false"
                layout="${JSONLayout}"
                />

```