Inconsistent date_range behavior when using DateOffset
#### A small, complete example of the issue

pd.date_range returns a different number of dates when used with different dates and the same DateOffset. Note that both dates are month end dates and that 2008 is a leap year (potential source of bug).

``` python
In [1]:
month = pd.to_datetime('2007-02-28')
pd.date_range(month, month + pd.DateOffset(months=12), freq='M')

Out[1]:
DatetimeIndex(['2007-02-28', '2007-03-31', '2007-04-30', '2007-05-31',
               '2007-06-30', '2007-07-31', '2007-08-31', '2007-09-30',
               '2007-10-31', '2007-11-30', '2007-12-31', '2008-01-31'],
              dtype='datetime64[ns]', freq='M')

In [2]:
month = pd.to_datetime('2007-01-31')
pd.date_range(month, month + pd.DateOffset(months=12), freq='M')

Out[2]:
DatetimeIndex(['2007-01-31', '2007-02-28', '2007-03-31', '2007-04-30',
               '2007-05-31', '2007-06-30', '2007-07-31', '2007-08-31',
               '2007-09-30', '2007-10-31', '2007-11-30', '2007-12-31',
               '2008-01-31'],
```

However, we see that using the to-be deprecated datetools gives consistent behavior:

``` python
In [2]:
month = pd.to_datetime('2007-02-28')
pd.date_range(month, month + pd.datetools.MonthEnd(12), freq='M')

Out[2]:
DatetimeIndex(['2007-02-28', '2007-03-31', '2007-04-30', '2007-05-31',
               '2007-06-30', '2007-07-31', '2007-08-31', '2007-09-30',
               '2007-10-31', '2007-11-30', '2007-12-31', '2008-01-31',
               '2008-02-29'], dtype='datetime64[ns]', freq='M')

In [3]:
month = pd.to_datetime('2007-01-31')
pd.date_range(month, month + pd.datetools.MonthEnd(12), freq='M')

Out[3]:
DatetimeIndex(['2007-01-31', '2007-02-28', '2007-03-31', '2007-04-30',
               '2007-05-31', '2007-06-30', '2007-07-31', '2007-08-31',
               '2007-09-30', '2007-10-31', '2007-11-30', '2007-12-31',
               '2008-01-31'], dtype='datetime64[ns]', freq='M')
```
#### Expected Output

``` python
In [1]:
month = pd.to_datetime('2007-02-28')
pd.date_range(month, month + pd.DateOffset(months=12), freq='M')

Out[2]:
DatetimeIndex(['2007-02-28', '2007-03-31', '2007-04-30', '2007-05-31',
               '2007-06-30', '2007-07-31', '2007-08-31', '2007-09-30',
               '2007-10-31', '2007-11-30', '2007-12-31', '2008-01-31',
               '2008-02-29'], dtype='datetime64[ns]', freq='M')
```
#### Output of `pd.show_versions()`

<details>
## INSTALLED VERSIONS

commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.19-29.55.amzn1.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 23.0.0
Cython: 0.24
numpy: 1.11.1
scipy: 0.17.1
statsmodels: 0.6.1
xarray: None
IPython: 4.2.0
sphinx: 1.4.1
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: 1.1.0
tables: 3.2.2
numexpr: 2.6.0
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.2
lxml: 3.6.0
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.13
pymysql: None
psycopg2: 2.6.2 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: 2.40.0
pandas_datareader: None
</details>
