Implement Subresource Integrity

Implemented response validation part of
https://w3c.github.io/webappsec-subresource-integrity/.
Implemented step eighteen of the main fetch. If a request has integrity
metadata, then following steps are performed
*Wait for response body
*If the response does not have a termination reason and response does not
match request’s integrity metadata, set response to a
network error.# Please enter the commit message for your changes. Lines starting

components/net/http_loader.rs

@@ -1378,7 +1378,7 @@ fn cors_check(request: Rc<Request>, response: &Response) -> Result<(), ()> {
     }
 
     // Step 6
-    let credentials = request.headers.borrow().get::<AccessControlAllowCredentials>().cloned();
+    let credentials = response.headers.get::<AccessControlAllowCredentials>().cloned();
 
     // Step 7
     if credentials.is_some() {