pandas2ri.py2ri(df) 
i want use pandas2ri.py2ri(df) ro replace r_dataframe = com.convert_to_r_dataframe(df)
for my data,i find the old function can work well but the pyr2 report error
 message like:

```
NotImplementedError                       
Traceback (most recent call last) <ipython-input-40-5278a5dfa2ad> in <module>()
     18 
     19 # r_dataframe = com.convert_to_r_dataframe(data)
---> 20 r_datafrmae=pandas2ri.py2ri(data)
     21 
     22 # type(r_dataframe)

/usr/local/lib/python2.7/dist-packages/singledispatch.pyc in wrapper(*args, **kw)
    208 
    209     def wrapper(*args, **kw):
--> 210         return dispatch(args[0].__class__)(*args, **kw)
    211 
    212     registry[object] = func

/usr/local/lib/python2.7/dist-packages/rpy2-2.7.2-py2.7-linux-x86_64.egg/rpy2/robjects/pandas2ri.pyc in py2ri_pandasdataframe(obj)
     42             od[name] = StrVector(values)
     43         else:
---> 44             od[name] = conversion.py2ri(values)
     45     return DataFrame(od)
     46 

/usr/local/lib/python2.7/dist-packages/singledispatch.pyc in wrapper(*args, **kw)
    208 
    209     def wrapper(*args, **kw):
--> 210         return dispatch(args[0].__class__)(*args, **kw)
    211 
    212     registry[object] = func

/usr/local/lib/python2.7/dist-packages/rpy2-2.7.2-py2.7-linux-x86_64.egg/rpy2/robjects/conversion.pyc in _py2ri(obj)
     58     (ri) objects.
     59     """
---> 60     raise NotImplementedError("Conversion 'py2ri' not defined for objects of type '%s'" % str(type(obj)))
     61 
     62 def _py2ro(obj):

NotImplementedError: Conversion 'py2ri' not defined for objects of type '<class 'pandas.core.series.Series'>'
```

the data type is pandas.core.frame.DataFrame,so i can not understand the error .
