title: DataFrame.registerTempTable sql failed
body: <p>I have dataframe in the following schema:</p>
  
  <pre><code>df.printSchema()
  //root
  // |-- FileItem: array (nullable = true)
  // |    |-- element: struct (containsNull = true)
  // |    |    |-- -created: string (nullable = true)
  // |    |    |-- -uid: string (nullable = true)
  // |    |    |-- Accessed: string (nullable = true)
  // |    |    |-- Changed: string (nullable = true)
  // |    |    |-- Created: string (nullable = true)
  </code></pre>
  
  <p>I register table FileItem</p>
  
  <pre><code>df.registerTempTable("FileItem")
  val newdf = sqlContext.sql("select FileItem.-uid from FileItem")
  </code></pre>
  
  <p>But error:</p>
  
  <pre><code>Exception in thread "main" java.lang.RuntimeException: [1.17] failure: ``*'' expected but `-' found
  
  select FileItem.-uid from FileItem
                  ^
  </code></pre>
  
  <p>Is there a way to get the column "-uid"?
  Thanks</p>