title: error when using `dataframemapper` class from pickle
body: <p>I am trying to save a <code>DataFramMapper</code> object to use on new data for a model.<br></p>
  
  <pre><code> mapper = DataFrameMapper([
          (['price', 'Argentina', 'Canada', 'Australia', 'barcat_numeric'], None),
          ('TTL',CountVectorizer( ngram_range=(1, 2))),
          ('BARCAT', CountVectorizer( ngram_range=(1, 2)))
      ])
      with open('company_dill.pkl', 'wb') as f:
          dill.dump(mapper, f)
  </code></pre>
  
  <p>when i read in the data: <br></p>
  
  <pre><code>with open('company_dill.pkl', 'rb') as f:
  mapper_v = dill.load(f)
  print(type(mapper_v))
  </code></pre>
  
  <p>output is: <br>
      
  but when i try using it i get:<br></p>
  
  <pre><code>---&gt; 20     X = mapper.transform(data_frame)
   21     return X
  
  C:\Users\eliav\Anaconda3\lib\site-
  packages\sklearn_pandas\dataframe_mapper.py in transform(self, X)
      269         extracted = []
      270         self.transformed_names_ = []
  --&gt; 271         for columns, transformers, options in self.built_features:
      272             input_df = options.get('input_df', self.input_df)
      273             # columns could be a string or list of
  
  TypeError: 'NoneType' object is not iterable
  </code></pre>
  
  <p>When i do not save to pickle it works fine, used both <code>pickle</code> and <code>dill</code></p>