[Immutablize#convert_value] reduce number of class comparisons

Signed-off-by: David Crosby <dcrosby@fb.com>

lib/chef/node/immutable_collections.rb

@@ -33,16 +33,21 @@ def safe_dup(e)
       end
 
       def convert_value(value)
-        # The order in this case statement is *important*.
-        # ImmutableMash and ImmutableArray should be tested first,
-        # as this saves unnecessary creation of intermediate objects
         case value
-        when ImmutableMash, ImmutableArray
-          value
         when Hash
-          ImmutableMash.new(value, __root__, __node__, __precedence__)
+          if ImmutableMash === value
+            # Save an object creation
+            value
+          else
+            ImmutableMash.new(value, __root__, __node__, __precedence__)
+          end
         when Array
-          ImmutableArray.new(value, __root__, __node__, __precedence__)
+          if ImmutableArray === value
+            # Save an object creation
+            value
+          else
+            ImmutableArray.new(value, __root__, __node__, __precedence__)
+          end
         else
           safe_dup(value).freeze
         end