title: input_file_name with geospark
body: <p><code>input_file_name</code> returns a blank when I try it with geospark and shp files.</p>
  <pre><code>var spatialRDD = new SpatialRDD[Geometry]
  spatialRDD = ShapefileReader.readToGeometryRDD(spark.sparkContext, config.shp())
  
  Adapter.toDf(spatialRDD,spark).
    withColumn(&quot;filename&quot;, input_file_name())
  </code></pre>
  <p>What's the tight way to do get file name in this case and why doesn't <code>input_file_name</code> work? I'm using <code>org.datasyslab.geospark</code> library. spark 2.2.</p>