flyspell.el (flyspell-large-region): Use extended character mode if defined (Bug...

flyspell.el (flyspell-large-region): Use extended character mode if defined (Bug#1339).

flyspell-large-region was not setting extended character mode for the
external ispell process. This was affecting ispell with 8bit charsets
not being the default, like in german.

Since extended character mode was not used, process used default 7bit
pseudo-charset resulting in some messages like

 -> rterbuch - 68: word not found

about words not found.

This may be related to (Bug#1339).