DataFrame.stack() with flat columns won't sort
#### Code Sample, a copy-pastable example if possible

```python
In [2]: df = pd.DataFrame(1, index=range(3), columns=[1, 3, 2])

In [3]: df.stack()
Out[3]: 
0  1    1
   3    1
   2    1
1  1    1
   3    1
   2    1
2  1    1
   3    1
   2    1
dtype: int64
```
#### Problem description

The docs state "The level involved will automatically get sorted.", and this is indeed what happens if ``df.columns`` is a ``MultiIndex``.

Related: #18310 (the opposite problem, when sorting shouldn't happen).

#### Expected Output

```
In [3]: df.stack()
Out[3]: 
0  1    1
   2    1
   3    1
1  1    1
   2    1
   3    1
2  1    1
   2    1
   3    1
dtype: int64
```

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

<details>

INSTALLED VERSIONS
------------------
commit: cfad581e9a6d35c7d05d2b1f34e4a19f7ee15cc6
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-3-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.22.0.dev0+151.gcfad581e9
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.7.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.0dev
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: None
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.6
lxml: None
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1


</details>
