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

Source/core/xml/XMLHttpRequest.cpp

Index: Source/core/xml/XMLHttpRequest.cpp
diff --git a/Source/core/xml/XMLHttpRequest.cpp b/Source/core/xml/XMLHttpRequest.cpp
index 2c97108fa97ab88f4ef97c45626aca8d53a694a7..dfbf72a1462aa5f32de8c3a1c34e392c8960bc1c 100644
--- a/Source/core/xml/XMLHttpRequest.cpp
+++ b/Source/core/xml/XMLHttpRequest.cpp
@@ -594,7 +594,7 @@ void XMLHttpRequest::send(const String& body, ExceptionCode& ec)
     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);