Fix: add 'application/zip' accept header

[zaerl]

The export call is missing the application/zip accept header. See Automattic/wp-calypso#59527 for further information.

What?

This PR add an Accept: application/zip to the export function.

Why?

When an accept header of */* is sent the browser can't download the file. This happens in production environments.

How?

Testing Instructions

1. Ensure to run a local production site at http://localhost:8888/wp-admin/ (see instructions here: https://github.com/WordPress/gutenberg/blob/trunk/docs/contributors/code/getting-started-with-code-contribution.md)
2. Select Appearence -> Editor
3. Select the top right menu
4. Select "Export"
5. Ensure that a valid .zip is exported

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @zaerl! In case you missed it, we'd love to have you join us in our Slack community, where we hold regularly weekly meetings open to anyone to coordinate with each other.

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[scruffian]

I can't replicate the issue, but this tests well and makes sense.

[github-actions]

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Kudos!

[mburridge]

Added the Needs Dev Note label in case this needs a dev note (either individual or as part of a "misc" dev note) for WP 6.1 release.