Auto merge of #15889 - ferjm:issue-14519-cors-preflight, r=avadacatavra

Allow for redirects after a CORS-preflight

~Continue the work done in #14811 and #15547~. And applies the Fetch [spec change](0d9a4db8bc) about [allowing redirects after CORS preflights](https://github.com/whatwg/fetch/issues/204).

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix ~#14787 and~ #14519
- [X] There are tests for these changes. Several WPTs go from FAIL to SUCCESS. The new expected FAILs for the cors/redirect-preflight.htm tests are caused by the spec change done [here](0d9a4db8bc), as suggested in the associated [issue](https://github.com/servo/servo/issues/14519#issuecomment-284384667).

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components/net/fetch/methods.rs

@@ -17,7 +17,7 @@ use hyper::mime::{Mime, SubLevel, TopLevel};
 use hyper::status::StatusCode;
 use mime_guess::guess_mime_type;
 use net_traits::{FetchTaskTarget, NetworkError, ReferrerPolicy};
-use net_traits::request::{RedirectMode, Referrer, Request, RequestMode, ResponseTainting};
+use net_traits::request::{Referrer, Request, RequestMode, ResponseTainting};
 use net_traits::request::{Type, Origin, Window};
 use net_traits::response::{Response, ResponseBody, ResponseType};
 use std::borrow::Cow;
@@ -240,7 +240,6 @@ pub fn main_fetch(request: Rc<Request>,
                  (!is_simple_method(&request.method.borrow()) ||
                   request.headers.borrow().iter().any(|h| !is_simple_header(&h)))) {
                 request.response_tainting.set(ResponseTainting::CorsTainting);
-                request.redirect_mode.set(RedirectMode::Error);
                 let response = http_fetch(request.clone(), cache, true, true, false,
                                           target, done_chan, context);
                 if response.is_network_error() {

components/net/http_loader.rs

@@ -746,11 +746,7 @@ fn http_redirect_fetch(request: Rc<Request>,
     request.redirect_count.set(request.redirect_count.get() + 1);
 
     // Step 7
-    let same_origin = if let Origin::Origin(ref origin) = *request.origin.borrow() {
-        *origin == request.current_url().origin()
-    } else {
-        false
-    };
+    let same_origin = location_url.origin()== request.current_url().origin();
     let has_credentials = has_credentials(&location_url);
 
     if request.mode == RequestMode::CorsMode && !same_origin && has_credentials {