servo: Merge #12467 - Add the append method for the Headers API (from jeenalee:jeena-headersAPI); r=jdm

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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](whatwg/fetch#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.

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Source-Repo: https://github.com/servo/servo
Source-Revision: 03fa7f0ba533acc44100639ad85625810618df3a

UltraBlame original commit: 9a892b1c7c7b04a342e32ba1e0a035a5cfcb328a

Author: marco-c

servo/components/script/dom/bindings/str.rs

@@ -162,6 +162,7 @@ Clone
 Eq
 PartialEq
 HeapSizeOf
+Debug
 )
 ]
 pub