REST API: Allow for multiple Vary: Origin headers in GET responses.

Simple fix, we pass `false` as the second parameter to the header function.

This is something that we added downstream of the 5.2.4 release, but we missed in 5.2/trunk.

Fixes #48309.
Props xknown, whyisjake.

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git-svn-id: https://core.svn.wordpress.org/trunk@46341 1a063a9b-81f0-0310-95a4-ce76da25c4cd