PeriodIndex becomes DatetimeIndex on multiple BDay resampling
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

series = pd.Series(1., pd.period_range('2015', '2016', freq='B'))
resampled = series.resample('3B').sum()

print type(series.index)
print type(resampled.index)
```
#### Problem description

A `PeriodIndex` when resampled by a multiple of BusinessDays becomes a `DatetimeIndex`.  

#### Expected Output
I would expect it to be a 3B `PeriodIndex`

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-36-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.24
numpy: 1.12.0
scipy: 0.18.1
statsmodels: 0.6.1
xarray: 0.8.2
IPython: 4.1.1
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: None
numexpr: 2.5
matplotlib: 1.5.1
openpyxl: 2.3.3
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: None
lxml: 3.5.0
bs4: 4.5.1
html5lib: None
httplib2: 0.9.2
apiclient: None
sqlalchemy: 1.0.12
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.45.0
pandas_datareader: None
</details>
