修复 get_k_data 方法

[yutiansut]

In [1]: from pytdx.hq import TdxHq_API

In [2]: api=TdxHq_API()

In [3]: with api.connect():
   ...:     print(api.get_k_data('600031', '2017-09-14', '2017-09-15'))
   ...:
            open  close  high   low       vol       amount        date    code
date
2017-09-14  7.81   7.75  7.86  7.71  397598.0  308731776.0  2017-09-14  600031
2017-09-15  7.76   7.69  7.77  7.66  314262.0  241509184.0  2017-09-15  600031

[yutiansut]

#58