Taxonomy: Update `register_taxonomy` hook to use `WP_Taxonomy` object.

Casts `WP_Taxonomy` to an array for passing to the `register_taxonomy` hook. This maintains backward compatibility with the processed arguments used prior to WordPress 4.7.

Fixes #38765.

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git-svn-id: https://core.svn.wordpress.org/trunk@39223 1a063a9b-81f0-0310-95a4-ce76da25c4cd