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Keep content-type negotiation in place as ensure_content_type! (#2910)

#2908 stopped `Grape::Middleware::Formatter#ensure_content_type` from
writing into the headers Hash it was handed, returning
`headers.merge(Rack::CONTENT_TYPE => ...)` instead. That copy runs on
almost every response, and the inline pair made it cost three
allocations rather than one: Ruby builds the one-pair Hash literal, then
copies it again converting the implicit keyword Hash into a positional
argument for `Hash#merge`, and `Rack::Headers#merge` dups the receiver
on top of that.

Measured over 2000 requests through a formatted endpoint:

    merge      19906.25 kB / 196,000 objects
    in place   18968.75 kB / 190,000 objects

Three objects per response on the hot path, for a header the caller is
about to send anyway. The negotiation goes back to writing into
`headers`, and the method carries a `!` so the mutation is visible at
the call site. `build_formatted_response` drops the `typed_headers`
local it only needed in order to hold the copy, so `headers` is still
never reassigned, which is what #2908 was after.

The trade is that a mounted plain Rack app returning a frozen or shared
headers Hash is written into again, as it was before #2908.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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