title: @RequestHeader mapped value contains header twice
body: <p>We are facing a super strange problem: in our endpoint:</p>
  
  <pre><code>@PostMapping(value = "/send_event_to_payment_process")
  @Async
  public void sendEvent(@Valid @RequestBody final SendEventRequestDto dto, @RequestHeader(value = TENANT) String foo) {
  </code></pre>
  
  <p>the mapped @RequestHeader variable foo contains the vaue twice joined with a ',' ("test,test"). If we read the header programmatically using the request context:</p>
  
  <pre><code>public void sendEvent(@Valid @RequestBody final SendEventRequestDto dto, @Context final HttpServletRequest request) {
  final String tenant = request.getHeader(TENANT);
  </code></pre>
  
  <p>we receive the proper value (only once: "test").</p>
  
  <p>Any clues what the problem might be?!</p>
  
  <p>Thank you! </p>