title: How to load field with null value from json as number in Dataframe
body: <p>One of the Json field (age below) meant to be a number represented as null is coming up as string in Dataframe printschema</p>
  
  <p>input json file</p>
  
  <pre><code>{"AGE":null,"NAME":"abc","BATCH":190}
  {"AGE":null,"NAME":"abc","BATCH":190}
  </code></pre>
  
  <p>Spark Code and output</p>
  
  <pre><code>val df = spark.read.json("/home/white/tmp/a.json")
  df.printSchema()
  df.show()
  
  *********************
  OUTPUT
  *********************
  root
   |-- BATCH: long (nullable = true)
   |-- AGE: string (nullable = true)
   |-- NAME: string (nullable = true)
  
  +-----+----+----+
  |BATCH|AGE|NAME|
  +-----+----+----+
  |  190|null| abc|
  |  190|null| abc|
  +-----+----+----+
  </code></pre>
  
  <p>I want age to be a long and currently I am achieving this by creating a new StructType with age field as Long and recreating the Dataframe as df.sqlContext.createDataFrame( df.rdd, newSchema ). Can I get this done while spark.read.json api directly?</p>