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

LayoutTests/platform/win/http/tests/xmlhttprequest/workers/methods-async-expected.txt

Index: LayoutTests/platform/win/http/tests/xmlhttprequest/workers/methods-async-expected.txt
diff --git a/LayoutTests/platform/win/http/tests/xmlhttprequest/workers/methods-async-expected.txt b/LayoutTests/platform/win/http/tests/xmlhttprequest/workers/methods-async-expected.txt
index a554c53473bc48b3da6a1f36a428cee052862c62..223ff1a408092e669a9139854ebae00da3c08b2a 100644
--- a/LayoutTests/platform/win/http/tests/xmlhttprequest/workers/methods-async-expected.txt
+++ b/LayoutTests/platform/win/http/tests/xmlhttprequest/workers/methods-async-expected.txt
@@ -3,15 +3,15 @@ GET and POST should not be the only HTTP methods implemented in XMLHttpRequest f
 GET(""): GET(0 bytes), Content-Type: null
 GET(null): GET(0 bytes), Content-Type: null
 GET("123"): GET(0 bytes), Content-Type: null
-POST(""): POST(0 bytes), Content-Type: application/xml
+POST(""): POST(0 bytes), Content-Type: text/plain;charset=UTF-8
 POST(null): POST(0 bytes), Content-Type: null
-POST("123"): POST(3 bytes), Content-Type: application/xml
-PUT(""): PUT(0 bytes), Content-Type: application/xml
+POST("123"): POST(3 bytes), Content-Type: text/plain;charset=UTF-8
+PUT(""): PUT(0 bytes), Content-Type: text/plain;charset=UTF-8
 PUT(null): PUT(0 bytes), Content-Type: null
-PUT("123"): PUT(3 bytes), Content-Type: application/xml
-DELETE(""): DELETE(0 bytes), Content-Type: application/xml
+PUT("123"): PUT(3 bytes), Content-Type: text/plain;charset=UTF-8
+DELETE(""): DELETE(0 bytes), Content-Type: text/plain;charset=UTF-8
 DELETE(null): DELETE(0 bytes), Content-Type: null
-DELETE("123"): DELETE(3 bytes), Content-Type: application/xml
+DELETE("123"): DELETE(3 bytes), Content-Type: text/plain;charset=UTF-8
 HEAD(""): HEAD(0 bytes), Content-Type: null
 HEAD(null): HEAD(0 bytes), Content-Type: null
 HEAD("123"): HEAD(0 bytes), Content-Type: null