pd.DataFrame.sub change the original dataframe order
#### Code Sample, a copy-pastable example if possible

```python
df1 = pd.DataFrame([[2], [4], [4], [3]])
df1.index = [0, 1, 2, 1]
df2 = pd.DataFrame([[10], [12]])
df2.index = [1, 2]
df3 = df1 - df2
print(df3.index)
# output: [0, 1, 1, 2]
```
#### Problem description
with minus by index, the output change my index from [0, 1, 2, 1] to [0, 1, 1, 2], which change the order of my original DataFrame.

#### Expected Output
[0, 1, 2, 1]

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Darwin
OS-release: 17.2.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: zh_CN.UTF-8
LOCALE: zh_CN.UTF-8

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 39.0.1
Cython: None
numpy: 1.14.2
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.3.1
sphinx: None
patsy: 0.5.0
dateutil: 2.7.2
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.0
openpyxl: 2.4.9
xlrd: 1.1.0
xlwt: None
xlsxwriter: None
lxml: 4.1.1
bs4: 4.6.3
html5lib: 0.9999999
sqlalchemy: 1.2.4
pymysql: 0.8.0
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
