save as: only re-guess the language if the file extension has changed, or 
it was previously an untitled document
http://bugs.activestate.com/show_bug.cgi?id=99822
rn=

(integrated from komodo (trunk) change 81203 by toddw)

git-svn-id: http://svn.openkomodo.com/repos/openkomodo/trunk@13357 b87d16e7-d29f-4ee1-b5ec-817d645d335f