title: Add column to existing MapType column
body: <p>I have a dataframe with several columns. One of these is a map (MapType).
  The keys inside this map-column differ from row to row. This means something like this is possible:</p>
  
  <pre><code>+----------+-----------------+
  |     col_1|            col_2|
  +----------+-----------------+
  |         7| key_1 -&gt; value_1|
  |         5| key_2 -&gt; value_2|
  |         4| key_3 -&gt; value_3|
  +----------+-----------------+
  </code></pre>
  
  <p>What i want to do is add the first column to this map-column to get something like:</p>
  
  <pre><code>+----------+-----------------------------+
  |     col_1|                        col_2|
  +----------+-----------------------------+
  |         7| key_1 -&gt; value_1, col_1 -&gt; 7|
  |         5| key_2 -&gt; value_2, col_1 -&gt; 5|
  |         4| key_3 -&gt; value_3, col_1 -&gt; 4|
  +----------+-----------------------------+
  </code></pre>
  
  <p>But i cant figure out how to add the first column to the map while perserving the individual keys inside the map-column.</p>