There are several exist places in Core that attempt to detect if a blog charset
is UTF-8. Each place attempts to perform the same check, except the logic is
spread throughout and there's no single method provided to make this
determination in a consistent way. The `_canonical_charset()` method exists,
but is marked private for use.
In this patch the new `unicode` module provides `is_utf8_charset()` as a method
taking an optional charset slug and indicating if it represents UTF-8,
examining all of the allowable variants of that slug. Associated code is
updated to use this new function, including `_canonical_charset()`. If no slug
is provided, it will look up the current `get_option( 'blog_charset' )`.
Finally, the test functions governing `_canonical_charset()` have been
rewritten as a single test with a data provider instead of as separate test
functions.
Developed in https://github.com/WordPress/wordpress-develop/pull/6535
Discussed in https://core.trac.wordpress.org/ticket/61182Fixes#61182.
Props dmsnell, jonsurrell.
Built from https://develop.svn.wordpress.org/trunk@58147
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