Added better defaults for composed_of, so statements like composed_of :time_zone, :mapping => %w( time_zone time_zone ) can be written without the mapping part (it's now assumed)

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@821 5ecf4fe2-1ee6-0310-87b1-e25e094e27de