title: Spark fillNa not replacing the null value
body: <p>I have the following dataset and its contain some null values, need to replace the null value using fillna in spark.</p>
  
  <p>DataFrame:</p>
  
  <pre><code>df = spark.read.format("com.databricks.spark.csv").option("header‌​","true").load("/sam‌​ple.csv")
  
  &gt;&gt;&gt; df.printSchema();
  root
   |-- Age: string (nullable = true)
   |-- Height: string (nullable = true)
   |-- Name: string (nullable = true)
  
  &gt;&gt;&gt; df.show()
  +---+------+-----+
  |Age|Height| Name|
  +---+------+-----+
  | 10|    80|Alice|
  |  5|  null|  Bob|
  | 50|  null|  Tom|
  | 50|  null| null|
  +---+------+-----+
  
  &gt;&gt;&gt; df.na.fill(10).show()
  </code></pre>
  
  <p>when i'll  give the na values it dosen't changed the same dataframe appeared again.</p>
  
  <pre><code>+---+------+-----+
  |Age|Height| Name|
  +---+------+-----+
  | 10|    80|Alice|
  |  5|  null|  Bob|
  | 50|  null|  Tom|
  | 50|  null| null|
  +---+------+-----+
  </code></pre>
  
  <p>tried create a new dataframe and store the fill values in dataframe but the result showing like unchanged.</p>
  
  <pre><code>&gt;&gt;&gt; df2 = df.na.fill(10)
  </code></pre>
  
  <p>how to replace the null values? please give me the possible ways by using fill na.
  Thanks in Advance.</p>