sym_diff failure on 3.4?
`test_symmetric_diff` is failing after trying out pandas on a fresh 3.4 pull.  It passes on 3.3 with the same numpy trunk version.  It seems to have to do with the nan section:

```
>>> from pandas import Index
>>> import numpy as np
>>> idx1 = Index([1, 2, np.nan])
>>> idx2 = Index([0, 1, np.nan])
>>> result = idx1.sym_diff(idx2)
>>> expected = Index([0.0, np.nan, 2.0, np.nan])  # oddness with nans
>>> nans = pd.isnull(expected)
>>> result
Float64Index([0.0, nan, nan, 2.0], dtype='object')
>>> expected
Float64Index([0.0, nan, 2.0, nan], dtype='object')
>>> nans
array([False,  True, False,  True], dtype=bool)
>>> result[nans]
Float64Index([nan, 2.0], dtype='object')
```

Version info (basically a fresh 3.4 build with only numpy dev installed):

```
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.4.0.candidate.1
python-bits: 32
OS: Linux
OS-release: 3.8.0-35-generic
machine: i686
processor: i686
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8

pandas: 0.13.1-282-g9564ead
Cython: None
numpy: 1.9.0.dev-2d6ea6e
scipy: None
statsmodels: None
IPython: None
sphinx: None
patsy: None
scikits.timeseries: None
dateutil: 2.2
pytz: 2013.9
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
bq: None
apiclient: None
rpy2: None
sqlalchemy: None
pymysql: None
psycopg2: None
```
