Fix: Bithumb Bad Request(Auth Data) - Convert header value to string

[channprj]

There's a problem in python code. It occurs Bad Request error because of Api-Sign value. Check below:

ExchangeNotAvailable: bithumb POST https://api.bithumb.com/info/balance 400 Client Error: Bad Request for url: https://api.bithumb.com/info/balance {"status":"5100","message":"Bad Request.(Auth Data)"}

I fixed python code before I read the contributing guide. Some commits will be overwritten automatically, but you can drop it. I checked python code what I fixed, but I can't checked transcompiled python code. Please review my code.

Anyway, I'll add another bithumb API if I have more time.
Thank you for your great project. 🎉 👍

[kroitor]

Thank you!