BUG: rolling.corr() produces wrong result with equal values
#### Code Sample, a copy-pastable example if possible

```python
s = pd.Series([1,1,2,2,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,5,0,0,0,7,0,0,0])
pd.rolling_corr(s,s,6)

```
#### Problem description

rolling_corr is producing the wrong result:
```
python
pd.rolling_corr(s,s,6)
```
0     NaN
1     NaN
2     NaN
3     NaN
4     NaN
5     1.0
6     1.0
7     1.0
8     1.0
9     0.0
10    0.0
11    0.0
12    0.0
13    0.0
14    0.0
15    0.0
16    0.0
17    0.0
18    0.0
19    0.0
20    0.0
21    0.0
22    0.0
23    0.0
24    0.0
25    0.0
26    1.0
27    1.0
28    1.0
29    1.0
30    1.0
31    1.0
32    1.0
33    1.0

This should have nan's instead of 0's for windows with static data.

#### Expected Output

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.1.35-pv-ts2
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.3
pytest: 3.0.7
pip: 9.0.1
setuptools: 36.4.0
Cython: 0.25.2
numpy: 1.11.3
scipy: 0.19.0
xarray: None
IPython: 5.3.0
sphinx: 1.6.2
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.3.0
numexpr: 2.6.2
feather: None
matplotlib: 1.5.1
openpyxl: 2.4.8
xlrd: 1.0.0
xlwt: None
xlsxwriter: 0.9.8
lxml: None
bs4: 4.5.3
html5lib: 0.999
sqlalchemy: 1.1.10
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
