1) Forward headers to summary endpoint to fetch proper displaytitle for
language variant
2) Add test using zh beta
3) TODO: Identify why language_variants_filter.js#12 is deleting
accept-language header and not forwarding it.

Bug: T231609

2) Pass vary and content-language to response

2) Remove unneeded check of shouldConvert
3) Test if accept-language is present in vary header

2) move related tests to language_variants
3) return req instead of undefined

2) Fix some linting issues
3) apply fetchSummary calls to check for summary key when hydrating
responses
4) Deep clone only needed params at language_variant_filter