qqq vs QQQ gives different result for barset

get_barset should not make a difference between capitalisation or not. Below and example via Ipython

```
In [8]: barset = api.get_barset('qqq', 'day', limit=5)

In [9]: barset
Out[9]: {'qqq': []}

In [10]: barset = api.get_barset('QQQ', 'day', limit=5)

In [11]: barset
Out[11]:
{'QQQ': [Bar({ 'c': 275.86,
'h': 277.71,
'l': 275.3719,
'o': 276.72,
't': 1601352000, …

```