Customize: Introduce a fix for themes that pass a stringable object through the `template_include` filter despite it being documented as only accepting a string.

This has historically worked until [61892] increased the strictness of the template file validation which dismissed any value of a type other than a string, which a stringable object is not.

Props dmsnell, westonruter.


git-svn-id: https://develop.svn.wordpress.org/trunk@61913 602fd350-edb4-49c9-b593-d223f7449a82