title: Convert Dataset of array into DataFrame
body: <p>Given <code>Dataset[Array[String]]</code>.
  In fact, this structure has a single field of array type.
  Is there any possibility to convert it into a <code>DataFrame</code> with each array item placed into a separate column?</p>
  
  <p>If I have <code>RDD[Array[String]]</code> I can achieve it in this way:</p>
  
  <pre><code>val rdd: RDD[Array[String]] = ???
  rdd.map(arr =&gt; Row.fromSeq(arr))
  </code></pre>
  
  <p>But surprisingly I cannot do the same with <code>Dataset[Array[String]]</code> – it says that there's no encoder for <code>Row</code>.</p>
  
  <p>And I cannot replace an array with <code>Tuple</code> or <code>case class</code> because the size of the array is unknown at compile time.</p>