Don't assume a file that is valid as UTF-8 is actually UTF-8 if we
have an information telling us otherwise in the document (a regex
match).

Also fix check for locale encoding.

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@5667 ea778897-0a13-0410-b9d1-a72fbfd435f5