As of [32484], `wp_dropdown_categories()` uses the `$value_field` value to
decide whether a given `<option>` should be 'selected'. However, `$value_field`
can refer to a value that is a string, such as a category's slug. This causes
problems when doing a loose comparison (`==`) with the value of the `'selected'`
parameter, which defaults to `0`, because when doing a loose comparison
between an integer and a string, PHP will cast the string to an integer. This
creates false matches, resulting in `<option>` elements getting a 'selected'
attribute incorrectly.
We address the issue by casting the comparison values to strings, and then
using the strict comparison operator `===`.
Fixes#33452 for trunk.
git-svn-id: https://develop.svn.wordpress.org/trunk@33681 602fd350-edb4-49c9-b593-d223f7449a82