title: fillna() method's behavior in Pandas -- does not obey to inplace argument - returns error on axis 1
body: <p>I am experimenting with the fillna() method.  I have created a small dataframe and two Series for that purpose:</p>
  
  <pre><code>        col1    col2    col3      col4
       0  NaN      NaN    3           4
       1  NaN      NaN    7           8
       2  9.0     10.0    11         12
  
       n1 = pd.Series([10, 20])
       n2 = pd.Series([30, 40, 50, 60])
       n2.index = list(df.columns.values)
  </code></pre>
  
  <p>When I try the command:</p>
  
  <pre><code>    df.fillna(n1, axis=0, inplace = True)
  </code></pre>
  
  <p>Nothing happens, the NaNs remain intact.  I would expect to see them replaced with the values 10 (col1) and 20 (col2).
  When I try</p>
  
  <pre><code>    df.fillna(n2, axis =1)
  </code></pre>
  
  <p>I get an error message:</p>
  
  <pre><code>   NotImplementedError: Currently only can fill with dict/Series column by column
  </code></pre>
  
  <p>Could you explain this behavior?  Your advice will be appreciated.</p>