Series accessed with timezoned timestamps inexplicably returns NaN
Somewhere between version 0.16.1 and 0.18.1 the behavior of Series[] called with an array of timestamps with timezones stopped working.

Is this expected behavior?

Here's our failing test case.
#### Code Sample, a copy-pastable example if possible

```
    import pytz
    import pandas as pp
    MTV_TZ = pytz.timezone('America/Los_Angeles')

    def D(*args):
      return pp.Timestamp(pp.datetime(*args), tz=MTV_TZ)
    series = pp.Series(
        index=[
            D(2011, 1, 1, 8),
            D(2011, 1, 1, 9),
            D(2011, 1, 1, 10),
            D(2011, 2, 2, 1),
            D(2011, 2, 2, 2),
            D(2011, 2, 2, 3),
        ], data=[1, 2, 3, 4, 5, 6])
    index = pp.Series(
        index=[D(2011, 1, 1), D(2011, 2, 2)],
        data=[D(2011, 1, 1, 9), D(2011, 2, 2, 3)])
    expected = pp.Series(
        index=[D(2011, 1, 1, 9), D(2011, 2, 2, 3)],
        data=[2, 6])
    # These are OK.
    print (series[index] == expected).all()
    print (series[(t for t in index)] == expected).all()
    print series[index[0]]#, 2)
    print series[index.values[0]]#, 2)
    print series[index.values[1]]#, 6
    # These fail. In 0.16.1 they passed
    print  (series[index.values] == expected).all()
    print (series[(t for t in index.values)] == expected).all()
    # Examining the output
    print 'SERIES'
    print series[index.values]
```
#### Expected Output

```
True
True
2
2
6
True
True
SERIES
2011-01-01 09:00:00-08:00    2
2011-02-02 03:00:00-08:00    6
dtype: int64
```
#### Actual Output

```
True
True
2
2
6
False
False
SERIES
2011-01-01 17:00:00   NaN
2011-02-02 11:00:00   NaN
dtype: float64
```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-83-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.16.1
nose: None
Cython: None
numpy: 1.9.2
scipy: 0.15.1
statsmodels: 0.6.1
IPython: 2.0.0
sphinx: None
patsy: 0.2.1
dateutil: 2.4.2
pytz: 2016.4
bottleneck: None
tables: 3.1.1
numexpr: 2.5
matplotlib: 1.3.0
openpyxl: None
xlrd: 0.9.3
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.3.2
html5lib: 1.0b8
httplib2: 0.9.2
apiclient: 1.3.1
sqlalchemy: None
pymysql: None
psycopg2: None
```
