When using 'show_in_menu' as a default value for 'show_in_admin_bar' in register_post_type(), cast to boolean instead of requiring a strict match.

props ipm-frommen.
fixes #30092.
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git-svn-id: http://core.svn.wordpress.org/trunk@30041 1a063a9b-81f0-0310-95a4-ce76da25c4cd