generate_symbol always uses numeric code

[tbradlo]

generate_symbol seems to be misimplemented now:

if symbol not in self.contracts: symbol = str(ib_contract.conId)

this condition is always true because self.contracts keys contains Exchange, e.g. 'BAS-EUR-STK.SMART'. while symbol does not have exchange information

[qdztxc]

find same issue. certainly a bug. At least not consistent with descripton: 委托、成交、持仓信息中的合约代码，默认采用数字代码。如果用户使用字符串代码订阅过行情，则使用字符串代码。

[vnpy]

已经修复，非常感谢