move all HashWithIndifferentAccess recursive enhancement over to the Util class

TODO: moving this two a class method on HashWithIndifferentAccess would really make the most sense, but there are two blockers:
- historically the method has operated out of Util. While totally ok, simply dedeferring to HashWithIndifferentAccess feels a bit awkward
- the contracts gem doesn’t support class method at this time. the assurance of frozen output is important considering the uses of the recursively_enhance method

middleman-core/lib/middleman-core/util.rb

@@ -80,8 +80,8 @@ module Middleman
     # @private
     # @param [Hash] data Normal hash
     # @return [Middleman::Util::HashWithIndifferentAccess]
-    FrozenDataStructure = Frozen[Or[HashWithIndifferentAccess, Array]]
-    Contract Maybe[Or[Array, Hash, HashWithIndifferentAccess]] => Maybe[FrozenDataStructure]
+    FrozenDataStructure = Frozen[Or[HashWithIndifferentAccess, Array, String, TrueClass, FalseClass, Fixnum]]
+    Contract Maybe[Or[String, Array, Hash, HashWithIndifferentAccess]] => Maybe[FrozenDataStructure]
     def recursively_enhance(data)
       if data.is_a? HashWithIndifferentAccess
         data
@@ -89,8 +89,10 @@ module Middleman
         HashWithIndifferentAccess.new(data)
       elsif data.is_a? Array
         data.map(&method(:recursively_enhance)).freeze
+      elsif data.frozen? || data.nil? || [::TrueClass, ::FalseClass, ::Fixnum].include?(data.class)
+        data
       else
-        nil
+        data.dup.freeze
       end
     end
 

middleman-core/lib/middleman-core/util/hash_with_indifferent_access.rb

@@ -19,7 +19,7 @@ module Middleman
         super()
 
         hash.each do |key, val|
-          self[key] = recursively_enhance(val)
+          self[key] = Util.recursively_enhance(val)
         end
 
         freeze
@@ -82,22 +82,6 @@ module Middleman
         end
       end
 
-      private
-
-      Contract Any => Frozen[Any]
-      def recursively_enhance(data)
-        if data.is_a? HashWithIndifferentAccess
-          data
-        elsif data.is_a? Hash
-          self.class.new(data)
-        elsif data.is_a? Array
-          data.map(&method(:recursively_enhance)).freeze
-        elsif data.frozen? || data.nil? || [::TrueClass, ::FalseClass, ::Fixnum].include?(data.class)
-          data
-        else
-          data.dup.freeze
-        end
-      end
     end
   end
 end