title: how to cast all columns of dataframe to string
body: <p>I have a mixed type dataframe. 
  I am reading this dataframe from hive table using 
  <code>spark.sql('select a,b,c from table')</code> command.</p>
  
  <p>Some columns are int , bigint , double and others are string. There are 32 columns in total.
  Is there any way in pyspark to convert all columns in the data frame to string type ?  </p>