title: Replace null values in Spark DataFrame
body: <p>I saw a solution here but when I tried it doesn't work for me.</p>
  
  <p>First I import a cars.csv file :</p>
  
  <pre><code>val df = sqlContext.read
                .format("com.databricks.spark.csv")
                .option("header", "true")
                .load("/usr/local/spark/cars.csv")
  </code></pre>
  
  <p>Which looks like the following :</p>
  
  <pre><code>+----+-----+-----+--------------------+-----+
  |year| make|model|             comment|blank|
  +----+-----+-----+--------------------+-----+
  |2012|Tesla|    S|          No comment|     |
  |1997| Ford| E350|Go get one now th...|     |
  |2015|Chevy| Volt|                null| null|
  </code></pre>
  
  <p>Then I do this : </p>
  
  <pre><code>df.na.fill("e",Seq("blank"))
  </code></pre>
  
  <p>But the null values didn't change. </p>
  
  <p>Can anyone help me ?</p>