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Avoid per-entry array allocation in Request#build_headers (#2770)

`build_headers` walked the env with `each_header.with_object(...)`. Because
`Enumerator#with_object` hands the block a single value plus the memo, the
two values `each_header` yields (`k`, `v`) get boxed into a throwaway
`[k, v]` Array on every iteration — which the `|(k, v), headers|` destructure
then immediately unpacks. That is one array allocated, packed, destructured,
and discarded per request header.

Drop `with_object` for a plain `each_header do |k, v|` block writing into a
pre-built `Grape::Util::Header`. `k` and `v` arrive as separate block args
(normal multi-value yield), so no array is boxed, and the accumulator is just
a closed-over local.

Output is byte-identical (`each_header` is `Rack::Request::Env#each_header`,
i.e. full env iteration; the `HTTP_` filter is unchanged). `build_headers`
runs lazily, only when an endpoint reads `headers`.

Measured on a request with ~30 headers: ~43% fewer objects (37 -> 21 per
call) and ~1.36x faster, with the array packing — not the Enumerator object
itself — accounting for the entire gap.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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