title: Combine PySpark DataFrame ArrayType fields into single ArrayType field
body: <p>I have a PySpark DataFrame with 2 ArrayType fields:</p>
  
  <pre><code>&gt;&gt;&gt;df
  DataFrame[id: string, tokens: array&lt;string&gt;, bigrams: array&lt;string&gt;]
  &gt;&gt;&gt;df.take(1)
  [Row(id='ID1', tokens=['one', 'two', 'two'], bigrams=['one two', 'two two'])]
  </code></pre>
  
  <p>I would like to combine them into a single ArrayType field:</p>
  
  <pre><code>&gt;&gt;&gt;df2
  DataFrame[id: string, tokens_bigrams: array&lt;string&gt;]
  &gt;&gt;&gt;df2.take(1)
  [Row(id='ID1', tokens_bigrams=['one', 'two', 'two', 'one two', 'two two'])]
  </code></pre>
  
  <p>The syntax that works with strings does not seem to work here:</p>
  
  <pre><code>df2 = df.withColumn('tokens_bigrams', df.tokens + df.bigrams)
  </code></pre>
  
  <p>Thanks!</p>