Correct the default Content-Type for XHR Post with string data as per XHR specification.

Source/core/xml/XMLHttpRequest.cpp

 /*
  *  Copyright (C) 2004, 2006, 2008 Apple Inc. All rights reserved.
  *  Copyright (C) 2005-2007 Alexey Proskuryakov <ap@webkit.org>
  *  Copyright (C) 2007, 2008 Julien Chaffraix <jchaffraix@webkit.org>
  *  Copyright (C) 2008, 2011 Google Inc. All rights reserved.
  *  Copyright (C) 2012 Intel Corporation
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
@@ skipping 576 matching lines @@
 }
 
 void XMLHttpRequest::send(const String& body, ExceptionCode& ec)
 {
     if (!initSend(ec))
         return;
 
     if (!body.isNull() && areMethodAndURLValidForSend()) {
         String contentType = getRequestHeader("Content-Type");
         if (contentType.isEmpty()) {
-            setRequestHeaderInternal("Content-Type", "application/xml");
+            setRequestHeaderInternal("Content-Type", "text/plain;charset=UTF-8");
         } else {
             replaceCharsetInMediaType(contentType, "UTF-8");
             m_requestHeaders.set("Content-Type", contentType);
         }
 
         m_requestEntityBody = FormData::create(UTF8Encoding().normalizeAndEncode(body, WTF::EntitiesForUnencodables));
         if (m_upload)
             m_requestEntityBody->setAlwaysStream(true);
     }
 
@@ skipping 641 matching lines @@
 {
     return &m_eventTargetData;
 }
 
 EventTargetData* XMLHttpRequest::ensureEventTargetData()
 {
     return &m_eventTargetData;
 }
 
 } // namespace WebCore