title: DataFrame drop column not working
body: <p>I have a DataFrame <code>df</code> with the following schema:</p>
  
  <pre><code>root
   |-- car: string (nullable = true)
   |-- person: struct (nullable = true)
   |    |-- age: long (nullable = true)
   |    |-- name: string (nullable = true)
  </code></pre>
  
  <p>Then I do: <code>new_df = df.drop("person.name")</code>. I also tried <code>df.drop(col("person.name"))</code> The schema of <code>new_df</code>:</p>
  
  <pre><code>root
   |-- car: string (nullable = true)
   |-- person: struct (nullable = true)
   |    |-- age: long (nullable = true)
   |    |-- name: string (nullable = true)
  </code></pre>
  
  <p>The schema of <code>new_df</code> has not changed. Any idea why?
  Assuming I want a final result with (person.age, car), how to do it?</p>