Resampler.nunique counting data more than once
xref addtl example in #13795 

Pandas `Resampler.nunique` appears to be putting the same data in multiple bins:

``` python
import pandas as pd

# Create a series with a datetime index
index = pd.date_range('1-1-2000', '2-15-2000', freq='h')
index2 = pd.date_range('4-15-2000', '5-15-2000', freq='h')
index3 = index.append(index2)
s = pd.Series(range(len(index3)), index=index3)

# Since all elements are unique, `count` and `nunique` should give the same result
count = s.resample('M').count()
nunique = s.resample('M').nunique()
```

In pandas 0.18.1 and 0.18.0 these don't give the same results, when they should

```
In [3]: count
Out[3]:
2000-01-31    744
2000-02-29    337
2000-03-31      0
2000-04-30    384
2000-05-31    337
Freq: M, dtype: int64

In [4]: nunique
Out[4]:
2000-01-31    337
2000-02-29    744
2000-03-31      0
2000-04-30    744
2000-05-31    337
Freq: M, dtype: int64
```

In pandas 0.17.0 and 0.17.1 (adjusting to old style resample syntax), the `nunique` one fails due to a "`ValueError: Wrong number of items passed 4, placement implies 5`" somewhere in the depths of `internals.py`. If I go back to 0.16.2, I do get the same result for each.

I'm not sure what's going on here. Since the `nunique` results sum to larger than the length, it appears data is being counted more than once.

---

``` python
In [19]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.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

pandas: 0.18.1
nose: None
pip: 8.1.2
setuptools: 23.0.0
Cython: None
numpy: 1.10.4
scipy: None
statsmodels: None
xarray: None
IPython: 4.2.0
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None
pandas_datareader: None
```
