meta key field in usermeta table should NOT use accent insensitive collations

[madhazelnut]

Looking at the latest string of vulnerability issues that came up related to the [Ultimate Member plugin](​https://wordpress.org/plugins/ultimate-member/) I discovered that the usermeta table has an accent insensitive collation for the meta_key field. This results in queries for wp_cãpăbilitiës to return the actual wp_capabilities row! See update_metadata() function in wp-includes/meta.php

Imagine the attack surface this brings. In fact, don't imagine, just look at the recent attacks in the wild.

Fix: use accent insensitive collations (or even ASCII BINARY)