Keep content-type negotiation in place as ensure_content_type!

[ericproulx]

Summary

#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 it showed up as a ~11% jump in total allocated memory on a real API profile.

The inline pair made it cost three allocations, not one:

expression	obj/call	breakdown
headers[K] = v (pre-#2908)	0	—
headers.merge(K => v) (#2908)	3	2 × Hash at the call site + 1 × Rack::Headers in Rack::Headers#merge
headers.merge(prebuilt)	1	1 × Rack::Headers
headers.dup then []=	1	1 × Rack::Headers

Two of the three are pure accident: Ruby builds the one-pair Hash literal, then copies it again converting the implicit keyword Hash into a positional argument for Hash#merge.

This restores the in-place write and renames the method ensure_content_type! 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 — headers is still never reassigned, which is what #2908 was actually after.

Perf

2000 requests through a formatted endpoint, whole-process allocations:

	allocated	objects
merge (master)	19906.25 kB	196,000
dup + []=	19281.25 kB	192,000
in place (this PR)	18968.75 kB	190,000

Three objects per response, for a header the caller is about to send anyway.

Trade-off

A mounted plain Rack app that returns a frozen or shared constant headers Hash is written into again, as it was before #2908. No spec covers that, and it was Grape's behaviour for years before #2908, so nothing regresses relative to 3.x — but it is the reason #2908 changed this, so calling it out.

CHANGELOG

#2908's entry claims ensure_content_type "no longer write[s] into the Hash they were given". That half is no longer true, so this PR corrects that line and drops the claim; the oneof half is untouched and still accurate. Neither shipped — both are unreleased under 4.0.0.

Test plan

• Full RSpec suite passes locally (2834 examples, 0 failures).
• RuboCop clean on the changed file (the 21 repo-wide offenses are pre-existing on master).
• CI green.

🤖 Generated with Claude Code

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