REST API: Fix error in _fields filtering logic where only one of several...

REST API: Fix error in _fields filtering logic where only one of several requested sibling properties would be included.

Props kadamwhite, TimothyBlynJacobs.
Fixes #48266.

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git-svn-id: https://core.svn.wordpress.org/trunk@46254 1a063a9b-81f0-0310-95a4-ce76da25c4cd