Indexing a time-series changes the index type
The following code changes the index type for my series:

``` python

import arrow
start_time = arrow.get(2014, 10, 15, hour = 19, tzinfo='US/Eastern')
end_time = start_time.replace(seconds=400)

ts = pd.date_range(start=start_time.isoformat(), end=end_time.isoformat(), freq='2200L')

my_series = pd.read_pickle(...)

my_series_sampled = my_series.ix[ts]
```

where the pickle file for `my_series` can be found on [this pickle file](https://drive.google.com/file/d/0B1_X48YFVS7STkh4bnB3ZndRYjg/view?usp=sharing).

More specifically:

For the input `my_series.head()`:

```
2014-10-14 19:00:05.861000-04:00    6.4400
2014-10-14 19:00:07.094000-04:00    6.4400
2014-10-14 19:00:07.109000-04:00    5.9584
2014-10-14 19:00:07.211000-04:00    6.2160
2014-10-14 19:00:07.410000-04:00    6.4400
Name: spend, dtype: float64 
```

the output: `my_series_sampled.head()` is:

```
1413414000000000000   NaN
1413414002200000000   NaN
1413414004400000000   NaN
1413414006600000000   NaN
1413414008800000000   NaN
Name: spend, dtype: float64
```

This is with `Python 3.4.1 :: Anaconda 2.1.0 (x86_64)` and:

```
Cython==0.21
DataShape==0.3.0
Flask==0.10.1
Jinja2==2.7.3
MarkupSafe==0.23
Pillow==2.5.1
PyYAML==3.11
Pygments==1.6
SQLAlchemy==0.9.7
Sphinx==1.2.3
Theano==0.6.0
Werkzeug==0.9.6
XlsxWriter==0.5.7
abstract-rendering==0.5.1
appnope==0.1.0
argcomplete==0.8.1
arrow==0.4.4
astropy==0.4.2
beautifulsoup4==4.3.2
binstar==0.7.1
bitarray==0.8.1
blaze==0.6.3
blz==0.6.2
bokeh==0.6.1
boto==2.34.0
cffi==0.8.6
colorama==0.3.1
configobj==5.0.6
cryptography==0.5.4
cytoolz==0.7.0
decorator==3.4.0
docutils==0.12
future==0.13.1
greenlet==0.4.4
h5py==2.3.1
ipython==2.2.0
itsdangerous==0.24
jdcal==1.0
jedi==0.8.1-final0
## FIXME: could not find svn URL in dependency_links for this package:
joblib==0.8.3-r1
llvmpy==0.12.7
lpsolve55==5.5.2.0
lxml==3.4.0
matplotlib==1.4.0
mock==1.0.1
multipledispatch==0.4.7
networkx==1.9.1
nltk==3.0.0
nose==1.3.4
numba==0.14.0
numexpr==2.3.1
numpy==1.9.0
openpyxl==1.8.5
pandas==0.15.0
parse==1.6.4
patsy==0.3.0
ply==3.4
psutil==2.1.1
psycopg2==2.5.4
py==1.4.25
pyOpenSSL==0.14
pycosat==0.6.1
pycparser==2.10
pycrypto==2.6.1
pycurl==7.19.5
pyflakes==0.8.1
pymc==3.0
pyparsing==2.0.1
pytest==2.6.3
python-dateutil==2.1
pytz==2014.7
pyzmq==14.3.1
qds-sdk==1.2.2
redis==2.9.1
requests==2.4.3
rope-py3k==0.9.4-1
runipy==0.1.1
scikit-image==0.10.1
scikit-learn==0.15.2
scipy==0.14.0
seaborn==0.5.dev
six==1.8.0
sockjs-tornado==1.0.1
spyder==2.3.1
statsmodels==0.5.0
sympy==0.7.5
tables==3.1.1
toolz==0.7.0
tornado==4.0.2
ujson==1.33
xlrd==0.9.3
```
