SettingWithCopyWarning while using .loc
#### Code Sample, a copy-pastable example if possible

```
df = pd.DataFrame(np.random.randn(6,4), columns=list('ABCD'))

# In ipython-notebook, I'm now just looking at the DF by calling it. This seems to create a view of the DF? If df is not called, the warning doesn't show. It will also show when doing x = df for example.
df

df = df.loc[df['A'] > 0]

# Now calling this, SettingWithCopyWarning shows suggesting to use .loc which is what I do?
df.loc[:, 'A'] = 1

```
#### Expected Output

What works without a warning is to use df.assign. But what about single row changes a la df.loc[1,'A'] = 1

Looking at the docs, df.loc seems to be the suggested way that should work fine.

Maybe I'm just missing something and the mistake is on my side. If so I'd be happy to know.
#### output of `pd.show_versions()`
## INSTALLED VERSIONS

commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Darwin
OS-release: 15.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: de_DE.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 20.3
Cython: 0.23.4
numpy: 1.10.4
scipy: 0.17.1
statsmodels: 0.6.1
xarray: None
IPython: 4.2.0
sphinx: 1.3.5
patsy: 0.4.0
dateutil: 2.5.1
pytz: 2016.2
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.4
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.12
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.39.0
pandas_datareader: 0.2.1
