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Copy response headers with merge! instead of merge (#2911)

`Grape::Util::Header.new.merge(headers)` appears twice on a request
path, and `merge` is a `dup` plus a `merge!` — so the Header built on
the same line is allocated only to be thrown away. `merge!` fills the
one already in hand and returns it, halving the cost:

* `Grape::API::Instance#call`, on every request when `cascade false` is
  set — the usual configuration for Grape mounted inside Rails.
* `Grape::Middleware::Error#rack_response`, on every error response.

Both copies stay. `headers` can come from a mounted Rack app, which is
free to hand back a frozen or shared Hash, and both call sites go on to
write into what they are given. The `cascade false` path is now pinned
by a spec that mounts a Rack app returning a frozen Hash; without the
copy it raises `FrozenError`.

`merge!` behaves the same on both halves of the supported Rack range:
Rack 2.2's `HeaderHash#merge!` is `other.each { |k, v| self[k] = v }`,
and Rack 3 aliases `merge!` to `update`. Both route through the
overridden `[]=`, so header names are still downcased. Note that the
shorter-looking `Grape::Util::Header.new(headers)` is not equivalent:
`Rack::Headers` subclasses `Hash`, so on Rack 3 the argument becomes
the default value and the result is empty.

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

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