REST API: Fix updating "multiple" meta keys with non-string values.

Previously, the REST API would end up deleting each row of metadata and recreating it unnecessarily. This was caused by a type mismatch where the metadata API would always return a string value, and the REST API operated on a typed value.

The REST API now applies the same sanitization and type casting for "multiple" meta keys and "single" meta keys.

Fixes #49339.
Props renathoc.

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git-svn-id: https://core.svn.wordpress.org/trunk@47716 1a063a9b-81f0-0310-95a4-ce76da25c4cd