title: Spark-xml creating `_VALUE` column which colflicts with other column with _value
body: <p>I am using Spark to process some datas stored in an XML file.
  I successfuly loaded my datas and printed the schema : </p>
  
  <pre><code>val df = spark.read
      .format("com.databricks.spark.xml")
      .option("rowTag","elementTag")
      .load(myPath+"/myfile.xml")
  
  df.printSchema
  </code></pre>
  
  <p>Which give me a result that look like this : </p>
  
  <pre><code>root
   |-- _id: string (nullable = true)
   |-- _type: string (nullable = true)
   |-- creationDate: struct (nullable = true)
   |    |-- _VALUE: string (nullable = true)
   |    |-- _value: string (nullable = true)
   |-- lastUpdateDate: struct (nullable = true)
   |    |-- _VALUE: string (nullable = true)
   |    |-- _value: string (nullable = true)
  </code></pre>
  
  <p>From this datas, I want to extract only certain fields , which should be easy with a 'select'. So I am doing the folowing request : </p>
  
  <pre><code>df.select("_id","creationDate._value","lastUpdateDate._value")
  </code></pre>
  
  <p>But I get the error :</p>
  
  <pre><code>org.apache.spark.sql.AnalysisException: Ambiguous reference to fields StructField(_VALUE,StringType,true), StructField(_value,StringType,true);
  </code></pre>
  
  <p>My problem is that spark sql is not case sensitive and my file contains field <code>_value</code> and <code>_VALUE</code> and I can't change my input file.</p>
  
  <p>Is there a way to solve this probleme with Spark?</p>