pandas.core.frame.DataFrame to pandas.core.series.Series warnings
#### Code Sample, a copy-pastable example if possible


```
df = pd.DataFrame(data = X)
r_dataframe = pandas2ri.py2ri(df)
MASS=importr('MASS')
result = MASS.mca(r_dataframe, nf = 2, abbrev = False)
print(type(result))
```
#### Problem description



feeding df from pandas: <class 'pandas.core.frame.DataFrame'>
Dataframe with no fancy index or column headings with binary 0 and 1 (forced to int)

script returns: <class 'rpy2.robjects.vectors.ListVector'>

but with warnings for every column:
UserWarning: Error while trying to convert the column "189". Fall back to string conversion. The error is: Conversion 'py2ri' not defined for objects of type '<class 'pandas.core.series.Series'>'


#### Expected Output

I worry about the warnings and can't understand why the data frame is converted to a series of series, how to stop it or solve it.

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

[paste the output of ``pd.show_versions()`` here below this line]

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.20.3
pytest: 3.2.2
pip: 9.0.1
setuptools: 36.5.0
Cython: 0.27
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: 6.2.0
sphinx: 1.6.4
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.0
lxml: 4.0.0
bs4: 4.6.0
html5lib: 1.0b10
sqlalchemy: 1.1.14
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None


</details>