Sanitizing username to prevent invalid email

In certain locales (e.g. nl_NL), special characters are used in names such as ç; when using these characters for generating email addresses, this will result in an invalid email address (e.g. valbinusgenaamdweissvonweissenlöw@example.net or sarah.ünal@example.com), which causes tests in my case to fail.
You need to namespace the constant (`\FILTER_SANITIZE_EMAIL `).

A unit tests would have shown the issue, please add one.