Make _mutations_prevent_wearing() properly account for the beak mutation

(which disallows hard helmets).

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@9116 c06c8d41-db1a-0410-9941-cceddc491573

Created by  dolorous  on February 17, 2009
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