BUG: Selecting a column from a data frame sometimes returns a series and sometimes returns a data frame
With pandas version 0.12:
In [3]: pandas.version.version
Out[3]: '0.12.0'
## Code to reproduce:

``` python
import collections
import pandas as pd
import numpy as np

cols = map(str, np.unique(np.random.randint(0, 1000000, 200)))
index = map(str, np.unique(np.random.randint(0, 1000000, 5000)))
df = pd.DataFrame(np.random.randint(0,5,(len(index), len(cols))),
                  index=index, columns=cols)

types = []
for _ in xrange(1000):
    subcols = np.random.choice(cols, 10)
    subindex = np.random.choice(index, 400)
    t = df.loc[subindex, subcols]
    if type(t[subcols[0]]) == pd.core.series.Series:
        types.append('Series')
    elif type(t[subcols[0]]) == pd.core.frame.DataFrame:
        types.append('Frame')
    else:
        raise Exception()

print collections.Counter(types)
```

---

About 20% of the time the result is a DataFrame and the other 80% is a Series
