reading a slice of a DataFrame seems to alter its dtypes, which further prevents repr()
Hi,

I noticed that after slicing, i couldn't have a clean repr() of my DataFrame due to an altered dtype. E.g.

``` python
import pandas as pd
print pd.__version__
pl = pd.DataFrame([range(10,15), range(11,16)])
_ = pl.loc[:,1]
print pl.dtypes
_ = pl.loc[:,1:3]
print pl.dtypes
print pl[3].dtype
```

The output being:

``` python
>>> import pandas as pd
>>> print pd.__version__
0.13.1
>>> pl = pd.DataFrame([range(10,15), range(11,16)])
>>> _ = pl.loc[:,1]
>>> print pl.dtypes
0    int64
1    int64
2    int64
3    int64
4    int64
dtype: object
>>> _ = pl.loc[:,1:3]
>>> print pl.dtypes
0    int64
1    int64
2    int64
3     None
4    int64
dtype: object
>>> print pl[3].dtype
int64
```

The "dtypes" list seems to be inconsitent. And, as a consequence the to_string() methods fail.
This happen on version 0.13.1, yet worked correctly on 0.12.0
Thanks
## INSTALLED VERSIONS

commit: None
python: 2.7.3.final.0
python-bits: 32
OS: Linux
OS-release: 3.2.0-58-generic-pae
machine: i686
processor: i686
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.13.1
Cython: 0.19.2
numpy: 1.8.1
scipy: 0.12.0
statsmodels: 0.5.0.dev-1bbd4ca
IPython: 2.0.0
sphinx: 1.1.3
patsy: 0.1.0
scikits.timeseries: None
dateutil: 2.2
pytz: 2014.2
bottleneck: None
tables: 2.3.1
numexpr: 1.4.2
matplotlib: 1.3.0
openpyxl: 1.6.1
xlrd: 0.6.1
xlwt: 0.7.2
xlsxwriter: None
sqlalchemy: None
lxml: None
bs4: None
html5lib: None
bq: None
apiclient: None
