Fix regression in username validation

Commit cdd7132ff9c3a8ac63621899f1e82bdd0ec58974 added logic to catch invalid database usernames,
but the regex it uses fails to match usernames with special characters that are properly quoted,
causing errors with usernames that used to work in versions < 3.0.0. This fixes the regex so that
if the username is quoted, anything is allowed between the quotes.

From the docs (http://dev.mysql.com/doc/refman/5.5/en/identifiers.html):
"Permitted characters in quoted identifiers include the full Unicode Basic Multilingual Plane (BMP),
except U+0000"