huobi pro "signature check failed"

```python
>>> huobipro.fetch_balance()
  File "/usr/local/lib/python3.6/site-packages/ccxt/async/huobipro.py", line 461, in fetch_balance
    }, params))
  File "/usr/local/lib/python3.6/site-packages/ccxt/async/base/exchange.py", line 102, in fetch2
    return await self.fetch(request['url'], request['method'], request['headers'], request['body'])
  File "/usr/local/lib/python3.6/site-packages/ccxt/async/base/exchange.py", line 129, in fetch
    self.handle_errors(http_status_code, text, url, method, self.last_response_headers, text)
  File "/usr/local/lib/python3.6/site-packages/ccxt/async/huobipro.py", line 720, in handle_errors
    raise ExchangeError(feedback)
```
I'm honestly not sure what this error means, but perhaps it should be an `AuthenticationError`. In any case, it's worth it to include it in the `this.exceptions` list.

[ci skip]
