REST API: Send a Vary: Origin header on GET requests.

Add this header on all GET requests to prevent cached requests.

Props darthhexx, davidbinda, nickdaugherty, whyisjake.

Built from https://develop.svn.wordpress.org/trunk@46478


git-svn-id: https://core.svn.wordpress.org/trunk@46276 1a063a9b-81f0-0310-95a4-ce76da25c4cd