Make parameter keep=False keep duplicates for nlargest/nsmallest
#### Code Sample, a copy-pastable example if possible

```python
>>> s = pd.Series([10,9,8,7,7,7,6])
>>> s.nlargest(4)
0    10
1     9
2     8
3     7
dtype: int64
```
#### Problem description
The docstrings list `False` as one of the possible argument values for `keep`.  pandas raises a `ValueError` when attempting to use this parameter. 

#### Expected Output
It would be nice to have nlargest work like this.

```python
>>> s.nlargest(4, keep=False)
0    10
1     9
2     8
3     7
4     7
5     7
```
#### Output of ``pd.show_versions()``

<details>
# Paste the output here pd.show_versions() here
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Darwin
OS-release: 15.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.2
pytest: 3.0.7
pip: 9.0.1
setuptools: 35.0.2
Cython: 0.25.2
numpy: 1.13.0
scipy: 0.19.0
xarray: None
IPython: 6.0.0
sphinx: 1.5.5
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.0
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: 0.3.0.post
</details>
