title: Interpret timestamp fields in Spark while reading json
body: <p>I am trying to read a pretty printed json which has time fields in it. I want to interpret the timestamps columns as timestamp fields while reading the json itself. However, it's still reading them as string when I <code>printSchema</code></p>
  
  <p>E.g. 
  Input json file - </p>
  
  <pre><code>[{
      "time_field" : "2017-09-30 04:53:39.412496Z"
  }]
  </code></pre>
  
  <p>Code - </p>
  
  <pre><code>df = spark.read.option("multiLine", "true").option("timestampFormat","yyyy-MM-dd HH:mm:ss.SSSSSS'Z'").json('path_to_json_file')
  </code></pre>
  
  <p>Output of <code>df.printSchema()</code> - </p>
  
  <pre><code>root
   |-- time_field: string (nullable = true)
  </code></pre>
  
  <p>What am I missing here?</p>