RecursionError when selecting single column from IntervalIndex
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame(
        np.ones((3, 4)), 
        columns=pd.IntervalIndex.from_breaks(np.arange(5)))
df[0.5]
df.loc[:, 0.5]
```
#### Problem description

Instead of returning the selected column, either calling `df[0.5]` or `df.loc[:, 0.5]` raises `RecursionError`.

The issue is in `frame.__getitem__` where `key in self.columns == False`. The code then correctly identifies the desired column by integer index using `self.columns.get_loc(key)`, but then goes on to call 
```python
data = self._take(indexer, axis=1)
data = data[key]
```
Because `self._take` returns a DataFrame, `data[key]` enters `frame.__getitem__` again and we are caught in an infinite loop.

#### Expected Output

The selected column as Series.

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

<details>

commit: None
python: 3.6.8.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 42 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None

pandas: 0.24.2
pytest: 4.3.1
pip: 19.0.3
setuptools: 40.8.0
Cython: 0.29.7
numpy: 1.16.2
scipy: 1.2.1
pyarrow: None
xarray: None
IPython: 7.4.0
sphinx: 1.8.5
patsy: 0.5.1
dateutil: 2.8.0
pytz: 2018.9
blosc: None
bottleneck: None
tables: 3.5.1
numexpr: 2.6.9
feather: None
matplotlib: 3.0.3
openpyxl: 2.6.1
xlrd: 1.2.0
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: 1.3.1
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
