[OKEx] Fix sandbox header exception with python
[OKEx] Fix sandbox header exception with python

Aiohttp can't serialize no-str `None` when setDemo has been called with False. 
Example here : https://github.com/Drakkar-Software/OctoBot-Trading/runs/4682784793?check_suite_focus=true#step:5:520
I think the best fix is to only add the header when using demo mode. There is no disable value in OKEx API doc, https://www.okex.com/docs-v5/en/#overview-demo-trading-services, it only describes that `x-simulated-trading: 1` should be added when using demo trading. 

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