title: convert columns of pyspark data frame to lowercase
body: <p>I have a dataframe in pyspark which has columns in uppercase like <code>ID</code>, <code>COMPANY</code> and so on</p>
  
  <p>I want to make these column names to <code>id</code> <code>company</code> and so on. Bacially convert all the columns to lowercase or uppercase depending on the requirement.</p>
  
  <p>I want to do in such away that the data types of the columns remain the same.</p>
  
  <p>How can we do that?</p>