Explicitly name supported collection types during normalize

Since an AbstractCollection can in principal contain any raw value, we
want to make sure that we only handle known values. Others will probably
still be catched by the fallback converter calls at the bottom of
AbstractCollection#normalize.

lib/rackstash/fields/abstract_collection.rb

@@ -115,7 +115,7 @@ module Rackstash
           return value
         when true, false, nil
           return value
-        when Rackstash::Fields::AbstractCollection
+        when Rackstash::Fields::Hash, Rackstash::Fields::Array
           return wrap ? value : value.raw
         when ::Hash
           hash = value.each_with_object(Concurrent::Hash.new) do |(k, v), memo|