title: Conversion from rpy2 dataframe to pandas dataframe not working for columns of string type
body: <p>If I try a conversion from pandas to R and back to pandas with a dataframe made only of numeric columns it goes ok, as can be seen below:</p>
  
  <pre><code>import pandas as pd
  df_py = pd.DataFrame({'col1':[1,2,3,4,5],
                        'col2':[1.0,2.0,3.0,4.0,5.0]})
  
  from rpy2.robjects.pandas2ri import py2ri
  df_r = py2ri(df_py)
  ri2py(df_r)
  </code></pre>
  
  <p>In this case, the result is:</p>
  
  <pre><code>  col1  col2
  0   1   1.0
  1   2   2.0
  2   3   3.0
  3   4   4.0
  4   5   5.0
  </code></pre>
  
  <p>Now if I include a third column of string type I get an error. Look below:</p>
  
  <pre><code>import pandas as pd
  df_py = pd.DataFrame({'col1':[1,2,3,4,5],
                        'col2':[1.0,2.0,3.0,4.0,5.0],
                        'col3':['a','b','c','d','e']})
  
  from rpy2.robjects.pandas2ri import py2ri
  df_r = py2ri(df_py)
  ri2py(df_r)
  </code></pre>
  
  <p>The error is:</p>
  
  <pre><code>/usr/local/lib/python3.6/dist-packages/numpy/core/_asarray.py in asarray(a, dtype, order)
       83 
       84     """
  ---&gt; 85     return array(a, dtype, copy=False, order=order)
       86 
       87 
  
  ValueError: Buffer for this type not yet supported.
  </code></pre>
  
  <p>Is this a issue in rpy2 or is there something that I'm doing wrong?</p>