Set response mime_type based on Content-Type
<!-- Please describe your changes on the following line: -->
Set the response's `mime_type` based on its `Content-Type` header whenever `Response::set_headers` is called. This ensures that `mime_type` is always consistent with `Content-Type`. The other reason for going with this approach is it relies on `extract_mime_type`: afbcbf75ea/components/script/dom/headers.rs (L260-L261)
Besides the consistency argument, I also expect that we'll update the `extract_mime_type` method to conform with its specification.
Another option I explored was adding a `set_mime_type` member to `Response` and calling that with the mime type from `Metadata::content_type` in `FetchResponseListener::process_response`. I decided against this since it expanded the interface of `Response` and didn't have the same benefits of the first option.
Fixes#24628.
---
<!-- 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
- [x] These changes fix#24628.
<!-- Either: -->
- [x] There are tests for these changes
<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
18ab860f4c