DataFrame.xs("key", axis=1, drop_level=False) still dropping level
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame([[1, 2, 3], [2, 4, 6]], columns=["type1_subtype1_subsubtype1", "type1_subtype1_subsubtype2", "type2_subtype1_subsubtype2"], index=["rowtype_rowsubtype1_rowlevel1", "row_rowlevel0_rowlevel1"])
df.columns = d.columns.str.split("_", expand=True).rename(["Sample_type", "Subtype", "SubSubtype"])
df.index = d.index.str.split("_", expand=True).rename(["Rowtype", "rowsubtype", "rowlevel"])
print("Incorrect behavior:")
print(df.xs("type1", axis=1, drop_level=False))
print("Expected output:")
print(df.transpose().xs("type1", drop_level=False).transpose())
```

#### Problem description
I checked if there was a previous issue about this in the issue tracker but my searches gave nothing similar. 

It appears that `drop_level=False` doesn't work as intended when taking a cross section along `axis=1`. Current workaround is to transpose the dataframe, then do the cross section, and then transpose it back again (as in my example above). 

#### Expected Output
```
Sample_type                         type1            
Subtype                          subtype1            
SubSubtype                    subsubtype1 subsubtype2
Rowtype rowsubtype  rowlevel                         
rowtype rowsubtype1 rowlevel1           1           2
row     rowlevel0   rowlevel1           2           4
```

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Linux
OS-release: 3.10.0-693.11.1.el7.x86_64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.1
pytest: 3.0.7
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 5.3.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.3.0
numexpr: 2.6.2
feather: 0.3.1
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
None
</details>

  