pandas.cut: the 'include_lowest' argument isn't behaving as documented
#### Code Sample

```python
import pandas as pd
import numpy as np

pd.cut(np.array([1, 7, 5, 4, 6, 3]), bins=[0, 3, 6, 8], include_lowest=True)
```

#### Problem description

Just by setting the `include_lowest` to `True` the data type of the interval changes from `int64` to `float64` and the first interval isn't left-inclusive. Here is the wrong output that you'll get:

```
[(-0.001, 3.0], (6.0, 8.0], (3.0, 6.0], (3.0, 6.0], (3.0, 6.0], (-0.001, 3.0]]
Categories (3, interval[float64]): [(-0.001, 3.0] < (3.0, 6.0] < (6.0, 8.0]]
```
#### Expected Output

```
[(0, 3], (6, 8], (3, 6], (3, 6], (3, 6], (0, 3]]
Categories (3, interval[int64]): [[0, 3] < (3, 6] < (6, 8]]
```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Darwin
OS-release: 17.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: 3.8.2
pip: 10.0.1
setuptools: 40.4.3
Cython: 0.28.5
numpy: 1.15.2
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 7.0.1
sphinx: 1.8.1
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.8
feather: None
matplotlib: 3.0.0
openpyxl: 2.5.8
xlrd: 1.1.0
xlwt: 1.2.0
xlsxwriter: 1.1.1
lxml: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.12
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
