to_csv() with a single level row MultiIndex does not produce the desired output

[joe1gi]

Code Sample, a copy-pastable example if possible

>>> import pandas as pd; 
>>> idx_1d = pd.MultiIndex(levels=[[1.0]], labels=[[0]], names=['x'])
>>> idx_2d = pd.MultiIndex(levels=[[1.], [2.]], labels=[[0], [0]], names=['x', 'y'])

# DataFrame
>>> print(pd.DataFrame({'data': [1]}, idx_2d).to_csv())
x,y,data
1.0,2.0,1

>>> print(pd.DataFrame({'data': [1]}, idx_1d).to_csv())
x,data
"('1.0',)",1

# Series
>>> print(pd.Series([1], idx_2d).to_csv())
1.0,2.0,1

>>> print(pd.Series([1], idx_1d).to_csv())
"('1.0',)",1

Problem description

The output for index.nlevels==1 should be the same as for index.nlevels==2, with one column less. That matches the output from other index types and was the output produced by pandas up to 0.20.3.

Expected Output

>>> print(pd.DataFrame({'data': [1]}, idx_1d).to_csv())
x,data
1.0,1

>>> print(pd.Series([1], idx_1d).to_csv())
1.0,1

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.0.final.0 python-bits: 64 OS: Windows OS-release: 7 machine: AMD64 processor: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel byteorder: little LC_ALL: None LANG: None LOCALE: None.None

pandas: 0.22.0
pytest: 3.2.1
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.11.3
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 5.1.0
sphinx: 1.5.1
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.4.2
numexpr: 2.6.1
feather: None
matplotlib: 2.0.0
openpyxl: 2.4.1
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: None
sqlalchemy: 1.1.5
pymysql: None
psycopg2: None
jinja2: 2.9.4
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[jreback]

cc @gfyoung

[gfyoung]

@joe1gi : Thanks for the report! I agree: the outputs should match. PR to patch is welcome!