REST API: Correctly map meta keys to field names.

This accidentally assumed $name was the same as $meta_key, which ruined the whole point of $name.

Props tharsheblows, joehoyle.
Fixes #38786.

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git-svn-id: https://core.svn.wordpress.org/trunk@39268 1a063a9b-81f0-0310-95a4-ce76da25c4cd