Refactor combine method, and update expected wpt results

Instead of creating an array with length of 1 of `b','`, then pushing
the first element of that array to `combined_value`, push a `b','`
directly to `combined_value`.

The web platform test for combining headers has been updated to reflect
the Fetch
spec (web-platform-tests/wpt#3646). The expected
web platform test results that will be affected by this change are updated.

components/script/dom/headers.rs

@@ -85,7 +85,7 @@ impl HeadersMethods for Headers {
         let mut combined_value: Vec<u8> = vec![];
         if let Some(v) = self.header_list.borrow().get_raw(&valid_name) {
             combined_value = v[0].clone();
-            combined_value.push(b","[0]);
+            combined_value.push(b',');
         }
         combined_value.extend(valid_value.iter().cloned());
         self.header_list.borrow_mut().set_raw(valid_name, vec![combined_value]);