Update balancer for recent gateway changes

[fengtality]

While we have fixed Uniswap and Uniswap-V3, these fixes have not yet been applied to the Balancer connector.

Modify the Balancer connector so that the AMM-ARB strategy can work with it. Since it should have the same responses and routes as the Uniswap connector, no client changes should be needed.

During testing, I encountered the following issues:

1. Approval bug

   Traceback (most recent call last):
   File "/Users/feng/Code/hummingbot/hummingbot/core/utils/async_utils.py", line 9, in safe_wrapper
   return await c
   File "/Users/feng/Code/hummingbot/hummingbot/connector/connector/balancer/balancer_connector.py", line 137, in auto_approve
   amount_approved = await self.approve_balancer_spender(token)
   File "/Users/feng/Code/hummingbot/hummingbot/connector/connector/balancer/balancer_connector.py", line 149, in approve_balancer_spender
   resp = await self._api_request("post",
   File "/Users/feng/Code/hummingbot/hummingbot/connector/connector/balancer/balancer_connector.py", line 589, in _api_request
   parsed_response = json.loads(await response.text())
   File "/Users/feng/anaconda3/envs/hummingbot/lib/python3.8/json/__init__.py", line 357, in loads
   return _default_decoder.decode(s)
   File "/Users/feng/anaconda3/envs/hummingbot/lib/python3.8/json/decoder.py", line 337, in decode
   obj, end = self.raw_decode(s, idx=_w(s, 0).end())
   File "/Users/feng/anaconda3/envs/hummingbot/lib/python3.8/json/decoder.py", line 355, in raw_decode
   raise JSONDecodeError("Expecting value", s, err.value) from None
   json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

2. Pool not found when it exists

   2021-08-27 07:24:31,138 - 56341 - hummingbot.connector.connector.balancer.balancer_connector - INFO - Unable to get price. No Pool Available

3 QA bug #139