[REVERT] WP_Theme_JSON: Prevent implicit coercion in to_ruleset

Reverts r62347. The fix the patch proposes is destined for 7.1. There is a general block for such patches until the 7.0 general release.

Props ramonopoly, andrewserong, westonruter.

See #64848.



git-svn-id: https://develop.svn.wordpress.org/trunk@62348 602fd350-edb4-49c9-b593-d223f7449a82