BUG: DataFrame.append seems to be sorting columns even when `sort=False`
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---


#### Code Sample, a copy-pastable example

```python
>>> import pandas as pd
>>> df = pd.DataFrame(
...             {
...                 0: [315.3324, 3243.32432, 3232.332, -100.32],
...                 1: [0.3223, 0.32, 0.0000232, 0.32224],
...             },
...             index=[7, 20, 11, 9],
...         )
>>> df
             0         1
7    315.33240  0.322300
20  3243.32432  0.320000
11  3232.33200  0.000023
9   -100.32000  0.322240
>>> series = pd.Series([10, 11, 23, 234, 13], index=[11, 12, 13, 44, 33])
>>> series
11     10
12     11
13     23
44    234
33     13
dtype: int64

>>> df.append(series, ignore_index=True)
           0         1     11    12    13    33     44
0   315.33240  0.322300   NaN   NaN   NaN   NaN    NaN
1  3243.32432  0.320000   NaN   NaN   NaN   NaN    NaN
2  3232.33200  0.000023   NaN   NaN   NaN   NaN    NaN
3  -100.32000  0.322240   NaN   NaN   NaN   NaN    NaN
4         NaN       NaN  10.0  11.0  23.0  13.0  234.0

>>> df.append(series, ignore_index=True, sort=True)
           0         1     11    12    13    33     44
0   315.33240  0.322300   NaN   NaN   NaN   NaN    NaN
1  3243.32432  0.320000   NaN   NaN   NaN   NaN    NaN
2  3232.33200  0.000023   NaN   NaN   NaN   NaN    NaN
3  -100.32000  0.322240   NaN   NaN   NaN   NaN    NaN
4         NaN       NaN  10.0  11.0  23.0  13.0  234.0

>>> df.append(series, ignore_index=True, sort=False)
           0         1     11    12    13    33     44
0   315.33240  0.322300   NaN   NaN   NaN   NaN    NaN
1  3243.32432  0.320000   NaN   NaN   NaN   NaN    NaN
2  3232.33200  0.000023   NaN   NaN   NaN   NaN    NaN
3  -100.32000  0.322240   NaN   NaN   NaN   NaN    NaN
4         NaN       NaN  10.0  11.0  23.0  13.0  234.0
>>> 

```

#### Problem description

The problem here is that even when `sort` is False, the output dataframe seems to have sorted columns(0, 1, 11, 12, 13, 33, 44), instead the desired order from what the docs say [here](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.append.html) should be (0, 1, 11, 12, 13, 44, 33).

#### Expected Output
```python
>>> df.append(series, ignore_index=True, sort=True)
           0         1     11    12    13    33     44
0   315.33240  0.322300   NaN   NaN   NaN   NaN    NaN
1  3243.32432  0.320000   NaN   NaN   NaN   NaN    NaN
2  3232.33200  0.000023   NaN   NaN   NaN   NaN    NaN
3  -100.32000  0.322240   NaN   NaN   NaN   NaN    NaN
4         NaN       NaN  10.0  11.0  23.0  13.0  234.0


>>> df.append(series, ignore_index=True, sort=False)
           0         1     11    12    13    44     33
0   315.33240  0.322300   NaN   NaN   NaN   NaN    NaN
1  3243.32432  0.320000   NaN   NaN   NaN   NaN    NaN
2  3232.33200  0.000023   NaN   NaN   NaN   NaN    NaN
3  -100.32000  0.322240   NaN   NaN   NaN   NaN    NaN
4         NaN       NaN  10.0  11.0  23.0  234.0  13.0
```



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


<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-76-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.5
numpy            : 1.17.5
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 47.3.1.post20200616
Cython           : 0.29.20
pytest           : 5.4.3
hypothesis       : 5.18.2
sphinx           : 3.1.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.15.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.17.1
pytables         : None
pytest           : 5.4.3
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : 0.49.1



</details>
