Bug in tseries.offsets.BDay
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
minRangeDate = '2015-12-28'
maxRangeDate = '2016-01-04'
nDays=0
for date in pd.date_range(start=minRangeDate, end=maxRangeDate):
    dateRange = (date.date(), (date+pd.tseries.offsets.BDay(nDays)).date())
    #dateRange = (date, date+pd.tseries.offsets.BDay(nDays))
    print(str(dateRange[0]), str(dateRange[1]))
```

gives

> 2015-12-28 2015-12-28
> 2015-12-29 2015-12-29
> 2015-12-30 2015-12-30
> 2015-12-31 2015-12-31
> 2016-01-01 2016-01-01
> **2016-01-02 2016-01-04**
> **2016-01-03 2016-01-04**
> 2016-01-04 2016-01-04

#### Problem description

Adding zero number of business days to a date should not change the date.

#### Expected Output

`(date+pd.tseries.offsets.BDay(0))==date` for any input date

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

<details>
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.10.0-693.17.1.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: en_US.utf-8
LANG: en_US.utf-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: 3.3.2
pip: 18.0
setuptools: 39.0.1
Cython: 0.27.3
numpy: 1.14.5
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.6
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 3.0.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.9999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>