Preserve flags properly when changing one monster type into another

(unless it's being used for polymorphing, of course).

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@3773 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on March 20, 2008
HOPP345EQLSF24XJYE3625VKU23PBKM5ZZKNCXBXUWYYORYPMDAAC
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