df.loc[:, 'col'] returning a view, but df.loc[df.index, 'col'] returning a copy
#### Code Sample, a copy-pastable example if possible

```
x = pd.DataFrame(zip(range(4), range(4)), columns=['a', 'b'])
print(x)
   a  b
0  0  0
1  1  1
2  2  2
3  3  3

q = x.loc[:, 'a']
q += 2
print(x)
   a  b
0  2  0
1  3  1
2  4  2
3  5  3

x = pd.DataFrame(zip(range(4), range(4)), columns=['a', 'b'])
print(x)
   a  b
0  0  0
1  1  1
2  2  2
3  3  3

q = x.loc[x.index, 'a']
q += 2
print(x)
   a  b
0  0  0
1  1  1
2  2  2
3  3  3
```


```
```
#### Problem description

[df.loc[:, 'col'] returning a view, but df.loc[df.index, 'col'] returning a copy, intended? how can I make sure it is returning a copy?]

#### Expected Output
I thought .loc[] was always returning a copy

pd.show_versions()

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 27.2.0
Cython: 0.24.1
numpy: 1.11.1
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.4.6
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: 1.1.0
tables: 3.2.2
numexpr: 2.6.1
matplotlib: 1.5.3
openpyxl: 2.3.2
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.3
lxml: 3.6.4
bs4: 4.5.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.42.0
pandas_datareader: None

</details>
