Google Finance DataReader returns columns with object type instead of float64
Hello,

Google Finance DataReader returns columns with object type instead of float64

```
In [112]: import pandas.io.data as web

In [113]: import datetime

In [114]: start = datetime.datetime(2010, 1, 1)

In [115]: end = datetime.datetime(2013, 1, 27)

In [116]: f=web.DataReader("F", 'google', start, end)

In [117]: f.dtypes
Out[117]:
Open       object
High       object
Low        object
Close     float64
Volume      int64
dtype: object
```
