Auto merge of #24669 - glowe:issue-24628/set-mime-type, r=jdm

Set response mime_type based on Content-Type

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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`: https://github.com/servo/servo/blob/afbcbf75eaa63ff0eec8fd3858e9155eb8dbadaa/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.

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- [x] These changes fix #24628.

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