merge error on Categorical column

```python

import pandas as pd
import numpy as np

df = pd.DataFrame({'a': np.random.randint(0, 100, 10)})
df['b'] = pd.cut(df['a'], np.arange(0, 110, 10))
pd.merge(df, df, on='b')

```
#### Problem description

pd.merge(df, df, on='b')

Out:
    a_x          b  a_y
0    66   (60, 70]   66
1    66   (60, 70]   68
2    68   (60, 70]   66
3    68   (60, 70]   68
4    93  (90, 100]   93
5    43   (40, 50]   43
6    43   (40, 50]   43
7    43   (40, 50]   43
8    43   (40, 50]   43
9    37   (30, 40]   37
10   37   (30, 40]   35
11   35   (30, 40]   37
12   35   (30, 40]   35
13    2    (0, 10]    2
14    2    (0, 10]    4
15    4    (0, 10]    2
16    4    (0, 10]    4
17   86   (80, 90]   86

df

Out:
    a          b
0  66   (60, 70]
1  93  (90, 100]
2  43   (40, 50]
3  37   (30, 40]
4  68   (60, 70]
5   2    (0, 10]
6  43   (40, 50]
7  86   (80, 90]
8  35   (30, 40]
9   4    (0, 10]

#### Expected Output

merge result's len should be equal to df

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.0
pytest: None
pip: 9.0.1
setuptools: 28.8.0
Cython: None
numpy: 1.13.1
scipy: 0.19.1
pyarrow: None
xarray: None
IPython: 6.0.0
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
