str method doesn't work after astype('object')
Given a small csv

```
City,State,Zip
Omaha,NE,68134
Chicago,IL,60290
Atlanta,GA,30303
```

then

```
df = pd.read_csv("test.csv")
df["Zip"].str.len()
```

not surprisingly yields 
_AttributeError: Can only use .str accessor with string values, which use np.object_ dtype in pandas_

this will not throw an error, but doesn't give me the expected result

```
df["Zip"] = df["Zip"].astype("object")
df["Zip"].str.len()

0  NaN
1  NaN
2  NaN
```

However, when I leverage the dtype arg in read_csv() it works

```
df = pd.read_csv("test.csv", dtype={"Zip" : "object"})
df["Zip"].str.len()

0   5
1   5
2   5
```

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.4.3.final.0
python-bits: 64
OS: Darwin
OS-release: 14.4.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.17.0
nose: None
pip: 7.1.2
setuptools: 18.3.2
Cython: 0.22
numpy: 1.10.0
scipy: 0.16.0
statsmodels: 0.6.1
IPython: 4.0.0
sphinx: 1.3.1
patsy: 0.3.0
dateutil: 2.4.2
pytz: 2015.6
blosc: None
bottleneck: None
tables: 3.1.1
numexpr: 2.3.1
matplotlib: 1.4.3
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: None
xlsxwriter: 0.6.7
lxml: 3.4.2
bs4: 4.3.2
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: 1.0.6
pymysql: None
psycopg2: 2.6 (dt dec pq3 ext)
```
