title: Convert XTS last business day of month index to last day of month
body: <p>I have a XTS object which has monthly data. However the date in the raw data is currently the last business day of the month, and I'd like it to be the last day of the month.</p>
  
  <p>Is there a way to convert the XTS object so that the index has the last day of the month?</p>
  
  <p>Thanks</p>