title: Merge two dataframes with column contains dictionary
body: <p>I have 2 dataframes questions, answers:</p>
  <p>Questions:</p>
  <pre><code>productId        question        questionId    total_answers
  ---------        --------        ----------    -------------
  147374           I wear Tan       4475521            2
                   Sand w/ Yellow
                   undertone ..
  </code></pre>
  <p>Answers:</p>
  <pre><code>productId       questionId     answerId      answer
  ---------       ----------     --------      ------
  147374           4475521        123456       answer1
  147374           4475521        654321       answer2
  </code></pre>
  <p>I want the result like this format:</p>
  <pre><code>productId     questionId      answers
  ---------     ----------      -------
  147374         4475521        [{'answerId':'123456','answer':'answer1'},
                                 {'answerId' :'654321','answer':'answer2'}]
  </code></pre>