contextengine: check for BOM when checking first_line
contextengine: check for BOM when checking first_line

This is needed so that we do not check 0==1 when we had a BOM that was
found in the first line (and therefore start_offset will be 1).

Fixes #168

GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview