The email addresses update fails if accentuated

If you try to update an accuentuated email address, it fails, since
MySQL (and the getByEmail method) isn't accentuated
characters-sensitive, but PHP is.

So we need to double-check if the new email address is really different
(and if it is really an already used one)
Merge pull request #1256 from samy-r/1.6

The email addresses update fails if accentuated