BUG: indexing in datetime IntervalIndex with duplicate values fails
#### Code Sample
```python
import pandas as pd
start_end_idx = pd.IntervalIndex.from_arrays(start, end)
start_end_idx.name = 'port_time'
mm = mm.set_index(['imo','port_id', start_end_idx])
global_bins = mm.index
global_bins.get_loc((8801527, 5610.0, pd.to_datetime('2016-10-16 16:57:44.316')))
```
```
/anaconda3/envs/mariquant/lib/python3.6/site-packages/pandas/core/indexes/interval.py in _engine(self)
    283     @cache_readonly
    284     def _engine(self):
--> 285         return IntervalTree(self.left, self.right, closed=self.closed)
    286 
    287     @property

pandas/_libs/intervaltree.pxi in pandas._libs.interval.IntervalTree.__init__()

KeyError: ('datetime64[ns]', 'right')
```
#### Problem description
It is impossible to use non unique IntervalIndex with datetimes as start-end points. Both index.get_loc and DataFrame.loc produce the same error in IntervalTree.

#### Expected Output

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]

INSTALLED VERSIONS
------------------
commit: None

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 38.5.1
Cython: None
numpy: 1.14.2
scipy: 1.0.0
pyarrow: None
xarray: 0.10.2
IPython: 6.2.1
sphinx: 1.7.1
patsy: 0.5.0
dateutil: 2.7.0
pytz: 2018.3
blosc: None
bottleneck: 1.2.1
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: 1.2.5
pymysql: None
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
