[SPARK-3778] newAPIHadoopRDD doesn't properly pass credentials for secure hdfs

https://issues.apache.org/jira/browse/SPARK-3778

This affects if someone is trying to access secure hdfs something like:
val lines = {
      val hconf = new Configuration()
      hconf.set("mapred.input.dir", "mydir")
      hconf.set("textinputformat.record.delimiter","\003432\n")
      sc.newAPIHadoopRDD(hconf, classOf[TextInputFormat], classOf[LongWritable], classOf[Text])
    }
This was just saying that the call to "new NewHadoopJob" adds the credentials to the conf passed in for you, so we don't need an explicit call to SparkHadoopUtil.get.addCredentials(jconf).

I can try to rephrase