title: scala dataframe to RDD[array[String]]
body: <p>How to convert dataframe with multiple columns
  I can get RDD[org.apache.spark.sql.Row], but I'd need something that I could use for org.apache.spark.mllib.fpm.FPGrowth, ei RDD[Array[String]]
  How to convert?</p>
  
  <pre><code>df.head
  org.apache.spark.sql.Row = [blabla,128323,23843,11.23,blabla,null,null,..]
  
  df.printSchema    
   |-- source: string (nullable = true)
   |-- b1: string (nullable = true)
   |-- b2: string (nullable = true)
   |-- b3: long (nullable = true)
   |-- amount: decimal(30,2) (nullable = true)
  and so on
  </code></pre>
  
  <p>Thanks</p>