Auto merge of #13004 - jeenalee:combine-headers, r=jdm

Update DOM headers `append` and `delete`

<!-- Please describe your changes on the following line: -->
Two changes are included in this PR:
1. A resolved TODO comment in `delete` is removed.
2. `append` method adds a space after a comma when combining header values. Expected wpt results are updated with this change.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script/dom/headers.rs

@@ -87,7 +87,7 @@ 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]);
@@ -107,8 +107,6 @@ impl HeadersMethods for Headers {
             return Ok(());
         }
         // Step 4
-        // TODO: Requires clarification from the Fetch spec:
-        // ... https://github.com/whatwg/fetch/issues/372
         if self.guard.get() == Guard::RequestNoCors &&
             !is_cors_safelisted_request_header(&valid_name, &b"invalid".to_vec()) {
                 return Ok(());