Email domain whitelist checks should be case-insensitive

[greatislander]

Within wpmu_validate_user_signup(), the user email domain is compared to an array of whitelisted email domains using in_array(). While these lines in is_email_address_unsafe() normalize the user email domain and array of banned email domains by making both lowercase before comparing, this code in wpmu_validate_user_signup() does not normalize case before comparing. This can lead to unexpected behaviour, as in_array() compares strings in a ​case-sensitive manner.

Given an email domain whitelist as follows…

$limited_email_domains = [ 'wordpress.org' ];

… a user who attempts to register with capital_P_dangit@WordPress.org will receive the following error:

Sorry, that email address is not allowed!

Domain names should be evaluated in a ​case-insensitive manner in this context.

[greatislander]

Patch from ​https://github.com/WordPress/WordPress/pull/337

[greatislander]

Add test, add true parameter to in_array()

[mnelson4]

These changes seem very straightforward and @greatislander provided good support for why the domains should be compared in a case-insensitive way.

[jeremyfelt]

In 42858:

Multisite: Use case-insensitive check on email domain whitelist.

Props greatislander.
Fixes #43148.