always set accept/accept-language headers when making a self-request

[Rich-Harris]

closes #7720. We previously decided not to automatically include request headers, but I think these exceptions are valid. Given that accept is used within Kit for content negotiation purposes, it makes sense to default it to */* if unspecified. Similarly, if accept-language were to be used for i18n purposes, it makes sense for it to match the initial page request.

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[benmccann]

awesome. thanks!