Make dungeon_terrain_changed() not destroy traps when the feature is changing

into a trap, so that Trowel cards and the "&L" wiz command can place traps.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@10580 c06c8d41-db1a-0410-9941-cceddc491573

Created by  zelgadis  on August 19, 2009
ZXQUQ5LBI7UYHZGECJQ673MLWGLVXIU3RZS6I4YIKY77H4YUSMYQC
Change contents