DataFrame.reset_index deletes index, does not all for ints as level arg
#### Code Sample, a copy-pastable example if possible
```python
import pandas as pd
import numpy as np
pd.__version__
```
  u'0.20.1'

```python
data = pd.DataFrame(np.random.randn(4,4), columns=['A', 'B', 'C', 'D']) 
data
```

<div>
<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th>A</th>
      <th>B</th>
      <th>C</th>
      <th>D</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th>0</th>
      <td>-0.134549</td>
      <td>2.352525</td>
      <td>0.481132</td>
      <td>-1.919506</td>
    </tr>
    <tr>
      <th>1</th>
      <td>1.980074</td>
      <td>0.720437</td>
      <td>0.410702</td>
      <td>-0.703470</td>
    </tr>
    <tr>
      <th>2</th>
      <td>-3.063166</td>
      <td>-0.781255</td>
      <td>0.270469</td>
      <td>-0.539081</td>
    </tr>
    <tr>
      <th>3</th>
      <td>-1.125265</td>
      <td>0.308374</td>
      <td>-0.166085</td>
      <td>-1.253959</td>
    </tr>
  </tbody>
</table>
</div>

```python
data.set_index(['A'], inplace=True)
data
```
<div>
<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th>B</th>
      <th>C</th>
      <th>D</th>
    </tr>
    <tr>
      <th>A</th>
      <th></th>
      <th></th>
      <th></th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th>-0.134549</th>
      <td>2.352525</td>
      <td>0.481132</td>
      <td>-1.919506</td>
    </tr>
    <tr>
      <th>1.980074</th>
      <td>0.720437</td>
      <td>0.410702</td>
      <td>-0.703470</td>
    </tr>
    <tr>
      <th>-3.063166</th>
      <td>-0.781255</td>
      <td>0.270469</td>
      <td>-0.539081</td>
    </tr>
    <tr>
      <th>-1.125265</th>
      <td>0.308374</td>
      <td>-0.166085</td>
      <td>-1.253959</td>
    </tr>
  </tbody>
</table>
</div>

```python
data.reset_index(level=['A'], inplace=True)
data
```
<div>

<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th>B</th>
      <th>C</th>
      <th>D</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th>0</th>
      <td>2.352525</td>
      <td>0.481132</td>
      <td>-1.919506</td>
    </tr>
    <tr>
      <th>1</th>
      <td>0.720437</td>
      <td>0.410702</td>
      <td>-0.703470</td>
    </tr>
    <tr>
      <th>2</th>
      <td>-0.781255</td>
      <td>0.270469</td>
      <td>-0.539081</td>
    </tr>
    <tr>
      <th>3</th>
      <td>0.308374</td>
      <td>-0.166085</td>
      <td>-1.253959</td>
    </tr>
  </tbody>
</table>
</div>


#### Problem description

between v0.19.2 and v0.20.1, the behavior of DataFrame.reset_index changed. 
With a single set index:
* It does not attempt to keep the column (essentially making drop=True always on)
* level=int no longer works (iterables work)

#### Expected Output

v0.19.2 results:

```python
data.reset_index(level=['A'], inplace=True)
data
```

<div>
<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th>A</th>
      <th>B</th>
      <th>C</th>
      <th>D</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th>0</th>
      <td>0.100442</td>
      <td>-0.620740</td>
      <td>-2.018020</td>
      <td>1.059871</td>
    </tr>
    <tr>
      <th>1</th>
      <td>-0.530272</td>
      <td>0.402598</td>
      <td>-1.453445</td>
      <td>-0.729623</td>
    </tr>
    <tr>
      <th>2</th>
      <td>-1.040126</td>
      <td>-0.536687</td>
      <td>-1.136123</td>
      <td>-0.748891</td>
    </tr>
    <tr>
      <th>3</th>
      <td>-0.269727</td>
      <td>0.182250</td>
      <td>0.847344</td>
      <td>0.785692</td>
    </tr>
  </tbody>
</table>
</div>


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

<details>

```python
import pandas as pd
import numpy as np
pd.show_versions()
```

    INSTALLED VERSIONS
    ------------------
    commit: None
    python: 2.7.13.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: None.None
    
    pandas: 0.19.2
    nose: None
    pip: 9.0.1
    setuptools: 35.0.2
    Cython: None
    numpy: 1.12.1
    scipy: None
    statsmodels: None
    xarray: None
    IPython: 5.3.0
    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: None
    html5lib: 0.999999999
    httplib2: None
    apiclient: None
    sqlalchemy: None
    pymysql: None
    psycopg2: None
    jinja2: 2.9.6
    boto: None
    pandas_datareader: None

</details>