[fix] add test cases and fix is_content_type_textual

[si3nloong]

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

Nothing much, just add more test cases

[changeset-bot]

🦋 Changeset detected

Latest commit: ce4d27e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[Rich-Harris]

thank you! mind fixing the lint errors?

for reference, it's helpful if you don't delete the pull request template — it includes a checklist that includes things like linting, and it's useful for maintainers to be able to understand the scope of a PR (whether it requires a version bump, etc)

[vercel]

@si3nloong is attempting to deploy a commit to the Svelte Team on Vercel.

A member of the Team first needs to authorize it.

[si3nloong]

@benmccann can you review it again?

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[Rich-Harris]

thanks!