Auto merge of #12467 - jeenalee:jeena-headersAPI, r=jdm

Add the append method for the Headers API

<!-- Please describe your changes on the following line: -->
This commit adds the append method for the Headers API. @malisas and I are both contributors.

There are a few TODOs related:
- The script needs to parse the header value for certain header names to decide the header group it belongs
- There are possible spec bugs that could change what a valid header value looks like (related: [issue page](https://github.com/whatwg/fetch/issues/332))

There are WPT tests already written for the Headers API, but they will fail as the Headers API is not fully implemented.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [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).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because tests for the Headers API already exists, but this commit does not implement the interface fully. The tests will fail.

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

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components/script/dom/xmlhttprequest.rs

@@ -25,6 +25,7 @@ use dom::document::DocumentSource;
 use dom::document::{Document, IsHTMLDocument};
 use dom::event::{Event, EventBubbles, EventCancelable};
 use dom::eventtarget::EventTarget;
+use dom::headers::is_forbidden_header_name;
 use dom::progressevent::ProgressEvent;
 use dom::xmlhttprequesteventtarget::XMLHttpRequestEventTarget;
 use dom::xmlhttprequestupload::XMLHttpRequestUpload;
@@ -409,21 +410,8 @@ impl XMLHttpRequestMethods for XMLHttpRequest {
                 // Step 5
                 // Disallowed headers and header prefixes:
                 // https://fetch.spec.whatwg.org/#forbidden-header-name
-                let disallowedHeaders =
-                    ["accept-charset", "accept-encoding",
-                    "access-control-request-headers",
-                    "access-control-request-method",
-                    "connection", "content-length",
-                    "cookie", "cookie2", "date", "dnt",
-                    "expect", "host", "keep-alive", "origin",
-                    "referer", "te", "trailer", "transfer-encoding",
-                    "upgrade", "via"];
-
-                let disallowedHeaderPrefixes = ["sec-", "proxy-"];
-
-                if disallowedHeaders.iter().any(|header| *header == s) ||
-                   disallowedHeaderPrefixes.iter().any(|prefix| s.starts_with(prefix)) {
-                    return Ok(())
+                if is_forbidden_header_name(s) {
+                    return Ok(());
                 } else {
                     s
                 }