fix(coinex): handleErrors should also check for "OK"

[roelvv]

The handleErrors method checks for "Ok", but some endpoints return "OK" instead. This causes an ExchangeError to be thrown for API responses that use "OK" but simply have no data.

An example is the pending positions endpoint (https://api.coinex.com/perpetual/v1/position/pending) while there are no open pending positions.
Also in their docs it isn't consistent if you look at the examples:

• "Ok" according to the docs: https://viabtc.github.io/coinex_api_en_doc/futures/#docsfutures001_http043_cancel_stop_order_by_client_id
• "OK" according to the docs: https://viabtc.github.io/coinex_api_en_doc/futures/#docsfutures001_http044_get_market_preference

And here is a live example for the V2 of it returning "OK": https://api.coinex.com/v2/maintain/info

[carlosmiei]

Hello @roelvv, thanks for fixing it, we will take a look and merge it shortly.