AJAX response is different from java/hibernate returned string

I've some problem with this simple server side method (java/hibernate):

@RequestMapping(value="/Prova" ,  method=RequestMethod.POST)
@Transactional( propagation = Propagation.REQUIRED, rollbackFor = Exception.class)
public ResponseEntity<String>  Prova(
        HttpServletRequest request, HttpServletResponse response
        ) throws Exception
{

    log.error("Sono qui");
    return new ResponseEntity<String>("pippotopolinopaperino",HttpStatus.OK);   

}

If the client does an AJAX request (I use ExtJs) the response is:

pippotopolinopa0015↵

WHY?

P.s sorry for my bad english

P.p.s I've tried also with:

@RequestMapping(value="/Prova" ,  method=RequestMethod.POST)
@Transactional( propagation = Propagation.REQUIRED, rollbackFor = Exception.class)
public @ResponseBody  String   Prova(
        HttpServletRequest request, HttpServletResponse response
        ) throws Exception
{

    log.error("Sono qui");
    return "pippotopolinopaperino"; 

}

I've solved in this way:

@RequestMapping(value="/Prova" ,  method=RequestMethod.POST)
@Transactional( propagation = Propagation.REQUIRED, rollbackFor = Exception.class)
public @ResponseBody  List<String>   Prova(
    HttpServletRequest request, HttpServletResponse response
    ) throws Exception
{    
  log.error("Sono qui");
  List<String> ret = new ArrayList<String>();
  ret.add("pippoPlutoTopolino");
  return ret;        
}

Or also:

@RequestMapping(value="/Prova2" ,  method=RequestMethod.POST)
@ResponseBody public StringBuffer Prova2(
        HttpServletRequest request
        ) throws Exception
{
  return new StringBuffer("pippoPlutoTopolino");  
}

But I don't understand why doesn't work returning a simple String.