Fix fetching about:blank

When fetching about:blank, response body should be the empty byte
sequence.
Spec: https://fetch.spec.whatwg.org/#concept-basic-fetch

Before this change, response body would be set to `ResponseBody::Empty`,
and then fetching would result in an infinite loop at step 19 in fn
`main_fetch` (methods.rs).

components/net/fetch/methods.rs

@@ -294,6 +294,7 @@ fn basic_fetch(request: Rc<Request>) -> Response {
                     response.headers.set(ContentType(Mime(
                         TopLevel::Text, SubLevel::Html,
                         vec![(Attr::Charset, Value::Utf8)])));
+                    *response.body.lock().unwrap() = ResponseBody::Done(vec![]);
                     response
                 },
                 _ => Response::network_error()

tests/unit/net/fetch.rs

@@ -94,6 +94,20 @@ fn test_fetch_response_body_matches_const_message() {
     };
 }
 
+#[test]
+fn test_fetch_aboutblank() {
+
+    let url = Url::parse("about:blank").unwrap();
+    let origin = Origin::Origin(url.origin());
+    let mut request = Request::new(url, Some(origin), false);
+    request.referer = Referer::NoReferer;
+    let wrapped_request = Rc::new(request);
+
+    let fetch_response = fetch(wrapped_request);
+    assert!(!fetch_response.is_network_error());
+    assert!(*fetch_response.body.lock().unwrap() == ResponseBody::Done(vec![]));
+}
+
 #[test]
 fn test_fetch_response_is_basic_filtered() {