DataFrame.join left_index right_index inverted
#### Code Sample, a copy-pastable example if possible

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

df_left = pd.DataFrame(data=['X'],columns=['C'],index=[22])
df_right = pd.DataFrame(data=['X'],columns=['C'],index=[999])
merge = pd.merge(df_left,df_right,on=['C'], left_index=True)

print merge.index

```
#### Problem description

The copied code print a DataFrame where the key is 999. As I understand from the documentation where left_index=True the keys from the left DataFrame should be used as join keys.
My output:
Int64Index([999], dtype='int64')
Expected output:
Int64Index([22], dtype='int64')

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-32-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.23.3
pytest: None
pip: 18.0
setuptools: 20.7.0
Cython: None
numpy: 1.15.0
scipy: None
pyarrow: None
xarray: None
IPython: 5.8.0
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: 1.0.5
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
