inf comparison doesn't emulate NaN with use_inf_as_null
#### Code Sample, a copy-pastable example if possible

```python
import numpy as np
import pandas as pd

with pd.option_context('mode.use_inf_as_null', True):
    s = pd.Series([np.nan, np.inf, 0])
    d = DataFrame(OrderedDict([
        ('s', s),
        ('repr(s)', s.map(repr)),
        ('s == s', s == s),
        ('s == np.inf', s == np.inf),
        ('s.values == np.inf', s.values == np.inf),
    ]))

d
=>

     s repr(s)  s == s  s == np.inf  s.values == np.inf
0  NaN     nan   False        False               False
1  NaN     inf    True        False                True
2  0.0     0.0    True        False               False
```
#### Problem description

I expected `s == s` to return False for inf values with `use_inf_as_null` mode on, or at least for `s == np.inf` to agree with `s.values == np.inf`.

#### Expected Output


```
     s repr(s)  s == s  s == np.inf  s.values == np.inf
0  NaN     nan   False        False               False
1  NaN     inf   False         True                True
2  0.0     0.0    True        False               False
```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Darwin
OS-release: 15.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.2
pytest: 3.1.2
pip: 9.0.1
setuptools: 33.1.1.post20170320
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 5.0.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.4.2
numexpr: 2.6.2
feather: 0.4.0
matplotlib: 2.0.2
openpyxl: 2.4.8
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 3.8.0
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.5
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
