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Match Accept media types case-insensitively (#2847)

Media types are case-insensitive (RFC 9110 section 8.3.1), but the registered
ones are spelled in lower case and matched literally, so a differently-cased
Accept header found nothing:

    Accept: TEXT/PLAIN                      -> served application/json
    Accept: APPLICATION/VND.TWITTER-V1+JSON -> api.version nil

Neither failed loudly. Content negotiation fell through to the default
format, and header versioning behaved as though no version had been asked
for, so the request was served by whichever version matched first -- the
client quietly got something other than what it asked for.

Three sites decided this, all comparing against lower-case registered types:
the formatter's Accept lookup, MediaType.best_quality_media_type, and the
vendor pattern in MediaType.parse / .match?. Down-case the incoming media
type at each. The vendor pattern stays lower-case, which is the case a
vendor and version are declared in and therefore compared in.

Grape already treats media types this way when deciding whether to escape an
error body (Middleware::Error#html_content_type?, from #2789).

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

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