Equality between DataFrames misbehaves if columns contain NaN
#### Code Sample, a copy-pastable example if possible

```python
In [2]: s = pd.DataFrame(-1, index=[1, np.nan, 2,],
   ...:                  columns=[3, np.nan, 1])
   ...: 

In [3]: s + s # good
Out[3]: 
       3.0  NaN    1.0
 1.0    -2    -2    -2
NaN     -2    -2    -2
 2.0    -2    -2    -2

In [4]: s == s # bad
Out[4]: 
       3.0 NaN    1.0
 1.0  True  NaN  True
NaN   True  NaN  True
 2.0  True  NaN  True
```

#### Problem description

While it is true that ``np.nan != np.nan``, pandas disregards this in indexes (indeed, ``s.loc[:, np.nan]`` works), so it should be coherent.
#### Expected Output

``` python
In [4]: s == s
Out[4]: 
       3.0  NaN   1.0
 1.0  True  True  True
NaN   True  True  True
 2.0  True  True  True
```

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

<details>

INSTALLED VERSIONS
------------------
commit: b45325e283b16ec8869aaea407de8256fc234f33
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-3-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.22.0.dev0+201.gb45325e28.dirty
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.7.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: None
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.6
lxml: None
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1


</details>
