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

LayoutTests/http/tests/security/mixedContent/insecure-sync-post-xhr-allowed-expected.txt

Index: LayoutTests/http/tests/security/mixedContent/insecure-sync-post-xhr-allowed-expected.txt
diff --git a/LayoutTests/http/tests/security/mixedContent/insecure-sync-post-xhr-allowed-expected.txt b/LayoutTests/http/tests/security/mixedContent/insecure-sync-post-xhr-allowed-expected.txt
index f3042115bece73b76eb429180bff461b63c88d91..2dd6d7723dbb639eb6c7b15a6698e3ac1c0e6f86 100644
--- a/LayoutTests/http/tests/security/mixedContent/insecure-sync-post-xhr-allowed-expected.txt
+++ b/LayoutTests/http/tests/security/mixedContent/insecure-sync-post-xhr-allowed-expected.txt
@@ -2,7 +2,5 @@ ALERT: Executing script in the child window.
 ALERT: XHR POST opened.
 CONSOLE MESSAGE: The page at https://127.0.0.1:8443/security/mixedContent/resources/frame-with-insecure-sync-xhr-post.html displayed insecure content from http://localhost:8000/xmlhttprequest/resources/access-control-allow-lists.php?origin=*&headers=Content-Type.
 
-CONSOLE MESSAGE: The page at https://127.0.0.1:8443/security/mixedContent/resources/frame-with-insecure-sync-xhr-post.html displayed insecure content from http://localhost:8000/xmlhttprequest/resources/access-control-allow-lists.php?origin=*&headers=Content-Type.
-
 ALERT: XHR POST sent.
 This test opens a HTTPS window that loads insecure data via XHR. We should trigger a mixed content callback because the main frame in the window is HTTPS but now has insecure data.