DataFrame astype may not work properly
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
df = pd.DataFrame({"a":[1,2,3], "b":["x", "y", "z"]})
df.astype({"a":str}, copy=True)
df.astype({"a":str}, copy=False)
print(df.dtypes)
```
#### Problem description

no matter what I give copy True or False. Dtype of \<column a\> will not be converted.
the output is:

> a     int64
> b    object


#### Expected Output

> a    object
> b    object

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.4.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.21.1
pytest: None
pip: 9.0.1
setuptools: 28.8.0
Cython: None
numpy: 1.13.3
scipy: None
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
