Series values cannot be accessed by numeric categorical index
#### Code Sample

```python
>>> s1 = pd.Series(['a', 'b', 'c'], index=pd.Series([1, 2, 3]))
>>> s1.get(3)
'c'

>>> s2 = pd.Series(['a', 'b', 'c'], index=pd.Series([1, 2, 3], dtype='category'))
>>> s2.get(3) is None
True
>>> s2.get(0)
'a'
```
#### Problem description

In my opinion behavior in the second case can be error-prone (when there is an overlap between positional index and categorical one) and inconvenient (forces to use positional index).

#### Expected Output

```python
>>> s2.get(3)
'c'
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Darwin
OS-release: 16.1.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.1
nose: None
pip: 9.0.1
setuptools: 27.2.0
Cython: None
numpy: 1.11.2
scipy: 0.18.1
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2016.7
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
</details>
