wrong index name during read_csv if using usecols
Hi,

If I have a test.csv file looks like below

```
SecId    Time               Price    P2    P3
10000   2013-5-11       100      10     1
500       2013-5-12     101      11     1
```

if I read in the above file with the following code

```
df =pd.read_csv('test.csv', usecols=['Time', 'Price'], parse_dates=True, index_col=0)
```

then `df.index.name` will be `SecId` instead of `Time`
