Fix ImageResponse headers merging (#67642)
### What?
`ImageResponse` merges default headers and developer-provided headers
incorrectly. The original implementation uses an object-based,
case-sensitive merging strategy, which results in developer-provided
header values being appended after the default header values by the
`Response` (or `Headers`) constructor.
### Why?
`ImageResponse` is used to generate image dynamically. Sometimes the
scenario would be the following. Given a URL without any timestamp, the
developer wants to generate an image depending on a mutable state. For
example, a profile image URL that is to be embedded on blogs by the end
user. In that case, the URL should be stable, and the `max-age` of the
image should be kept short enough because the end user might change
their profile. If the developer passes in a `Cache-Control` header with
any object key other than the fully lower-cased `cache-control`, the
developer's `Cache-Control` value gets **appended** after the default
`Cache-Control` value.
### How?
This PR fixes this by respecting developer-provided headers and
overwriting default ones if the developer has explicitly specified one.
Fixes #67641
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Co-authored-by: JJ Kasper <jj@jjsweb.site>
