Index.isin() always True for nans
This looks like a regression to me, the following code used to print `[ True]`, which I think is the correct behaviour, but prints `[False]` in recent development versions (0.13.1.dev).

```
import numpy as np
import pandas as pd

i = pd.Index([np.nan])
print i.isin({0})  # prints [ True], should be [False]
```

I do not think the following is relevant but just in case:

```
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-24-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: es_ES.UTF-8

pandas: 0.13.1.dev
nose: 1.3.1
Cython: None
numpy: 1.8.1
scipy: 0.13.3
statsmodels: 0.6.0.dev-Unknown
IPython: 3.0.0-dev
sphinx: None
patsy: 0.2.1
scikits.timeseries: None
dateutil: 1.5
pytz: 2012c
bottleneck: 0.8.0
tables: 3.1.1
numexpr: 2.2.2
matplotlib: 1.3.1
openpyxl: 1.7.0
xlrd: 0.9.2
xlwt: 0.7.5
xlsxwriter: 0.5.2
lxml: 3.3.3
bs4: 4.2.1
html5lib: 0.999
bq: None
apiclient: None
rpy2: None
sqlalchemy: None
pymysql: None
psycopg2: 2.4.5 (dt dec mx pq3 ext)
```
