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

LayoutTests/http/tests/xmlhttprequest/methods-async-expected.txt

Index: LayoutTests/http/tests/xmlhttprequest/methods-async-expected.txt
diff --git a/LayoutTests/http/tests/xmlhttprequest/methods-async-expected.txt b/LayoutTests/http/tests/xmlhttprequest/methods-async-expected.txt
index 2d2e6a761c3f6f81f9197636ed0338d00bfea674..6ce75ff5e3a3d8b9693032aa0b684333053039d6 100644
--- a/LayoutTests/http/tests/xmlhttprequest/methods-async-expected.txt
+++ b/LayoutTests/http/tests/xmlhttprequest/methods-async-expected.txt
@@ -3,21 +3,21 @@ Tests for bug 3812 - GET and POST should not be the only HTTP methods implemente
 GET(""): GET(null bytes), Content-Type: null
 GET(null): GET(null bytes), Content-Type: null
 GET("123"): GET(null 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(null 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
-WKFOOBAR(""): WKFOOBAR(0 bytes), Content-Type: application/xml
+WKFOOBAR(""): WKFOOBAR(0 bytes), Content-Type: text/plain;charset=UTF-8
 WKFOOBAR(null): WKFOOBAR(null bytes), Content-Type: null
-WKFOOBAR("123"): WKFOOBAR(3 bytes), Content-Type: application/xml
-SEARCH(""): SEARCH(0 bytes), Content-Type: application/xml
+WKFOOBAR("123"): WKFOOBAR(3 bytes), Content-Type: text/plain;charset=UTF-8
+SEARCH(""): SEARCH(0 bytes), Content-Type: text/plain;charset=UTF-8
 SEARCH(null): SEARCH(null bytes), Content-Type: null
-SEARCH("123"): SEARCH(3 bytes), Content-Type: application/xml
+SEARCH("123"): SEARCH(3 bytes), Content-Type: text/plain;charset=UTF-8