XMLRPC: Revert [35509] which caused a change of behviour in at least one XMLRPC client.

XMLRPC has many quirks in it's operation, #16980 being just one of the many, #35185 just became yet another quirk.

See #16980.
Fixes #35185.

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git-svn-id: https://core.svn.wordpress.org/trunk@36098 1a063a9b-81f0-0310-95a4-ce76da25c4cd