result from groupby / nlargest with data frame with one row does not include the groupby key in the resulting index
#### Code Sample, a copy-pastable example if possible

```python
In [1]: df = pandas.DataFrame([["Dog", 1], ["Dog", 2]], columns=["animal", "value"])

In [2]: df.groupby("animal").value.nlargest(5)
Out[2]: 
animal   
Dog     1    2
        0    1
Name: value, dtype: int64
In [3]: df = pandas.DataFrame([["Dog", 1]], columns=["animal", "value"])
In [4]: df.groupby("animal").value.nlargest(5)
Out[4]: 
0    1
Name: value, dtype: int64

```
#### Problem description


#### Expected Output

```python
In [3]: df = pandas.DataFrame([["Dog", 1]], columns=["animal", "value"])
In [4]: df.groupby("animal").value.nlargest(5)
Out[4]: 
animal
Dog      0    1
Name: value, dtype: int64
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.2
nose: None
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.12.1
scipy: None
statsmodels: None
xarray: None
IPython: 6.0.0
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.5.3
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
boto: None
pandas_datareader: None
</details>
