DataFrame.corr() throws error on simple data
```
df3 = DataFrame({"a":[1,2,3,4], "b":[1,2,3,4]})
df3
   a  b
0  1  1
1  2  2
2  3  3
3  4  4
df3.corr()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "C:\portabel\Python27\lib\site-packages\pandas\core\frame.py", line 3867, in corr
    correl = lib.nancorr(mat)
  File "moments.pyx", line 284, in pandas.lib.nancorr (pandas\src\tseries.c:67792)
ValueError: Buffer dtype mismatch, expected 'float64_t' but got 'long long'
```
