BUG: incorrect output of first('1M') in case if first index is the last day of the month
#### Code Sample

```python
x = pd.Series(1, index=pd.bdate_range('2010-03-31', periods=100))
print(x.first('1M'))  # Returns March and April
print(x.first('2M'))  # Returns March, April and May
# etc...
```
#### Problem description

In case if the first index of the series falls on the last day of the month, `Series.first('1M')` returns two first months (this one day + the next month).

If the first value is not on the last day of the month - result is correct:

```python
x = pd.Series(1, index=pd.bdate_range('2010-03-30', periods=100))
print(x.first('1M'))  # Returns only March
print(x.first('2M'))  # Returns March and April
# etc...
```

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

<details>


INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.1.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.3
numpy            : 1.17.2
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 41.4.0
Cython           : None
pytest           : 5.2.1
hypothesis       : None
sphinx           : 2.2.0
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.1
html5lib         : None
pymysql          : 0.9.3
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.8.0
pandas_datareader: None
bs4              : 4.6.3
bottleneck       : None
fastparquet      : 0.1.6
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.0.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.12.1
pytables         : None
s3fs             : 0.3.4
scipy            : 1.3.1
sqlalchemy       : 1.3.8
tables           : 3.4.4
xarray           : None
xlrd             : 1.1.0
xlwt             : None
xlsxwriter       : None


</details>
