IntervalIndex level in MultiIndex does not work as expected
I am trying to index data frame with a `MultiIndex` with constituent `IntervalIndex`
#### Example
```python
ii = pd.IntervalIndex.from_arrays([1, 5, 2, 6, 3, 7], [4, 8, 5, 9, 6, 10])
mi = pd.MultiIndex.from_arrays([["aa", "aa", "bb", "bb", "cc", "cc"], ii])
data = [(1, 2), (3, 4), (5, 6), (7, 8), (9, 10), (11, 12)]
df = pd.DataFrame(data, columns=['k', 'l'], index=mi)
df.loc[("bb", 3)]
```
Above code throws `TypeError` as follows:
```python
TypeError: cannot do label indexing on <class 'pandas.core.indexes.base.Index'> with these indexers [3] of <class 'int'>
```
I was expecting above to produce same result as:
```python
df.loc[("bb")].loc[3]
# k    5
# l    6
# Name: (2, 5], dtype: int64
```
Is this a bug or I'm not using it properly? (newbie here)
#### Version Details:
```
python: Python 3.6.7 | packaged by conda-forge | (default, Nov 21 2018, 03:09:43)
pandas: '0.23.4'
```