pd.tseries.frequencies.is_subperiod(x, x) is inconsistent
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

In [6]: pd._libs.tslibs.frequencies.is_subperiod('D', 'D')
Out[6]: True

In [7]: pd._libs.tslibs.frequencies.is_subperiod('M', 'M')
Out[7]: False
```
#### Problem description

When source == target, shouldn't pd._libs.tslibs.frequencies.is_subperiod(source, target) always return True?

#### Expected Output

```python
import pandas as pd

In [6]: pd._libs.tslibs.frequencies.is_subperiod('D', 'D')
Out[6]: True

In [7]: pd._libs.tslibs.frequencies.is_subperiod('M', 'M')
Out[7]: True
```

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

<details>
In [12]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-101-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.3
pytest: 3.1.3
pip: 9.0.1
setuptools: 37.0.0
Cython: 0.27.3
numpy: 1.13.1
scipy: 1.0.0
xarray: None
IPython: 6.1.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
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
pandas_gbq: None
pandas_datareader: None
</details>
