Revert "ActiveModel::Attribute: elide dup for immutable types"

Author: jhawthorn

activemodel/lib/active_model/attribute.rb

@@ -96,14 +96,6 @@ def with_type(type)
       end
     end
 
-    def deep_dup # :nodoc:
-      if @type.mutable?
-        dup
-      else
-        self # If the underlying type is immutable we can get away with not duping
-      end
-    end
-
     def type_cast(*)
       raise NotImplementedError
     end

activemodel/lib/active_model/attribute/user_provided_default.rb

@@ -26,10 +26,6 @@ def with_type(type)
         self.class.new(name, user_provided_value, type, original_attribute)
       end
 
-      # Can't elide dup when a default is provided.
-      # See Attribute#deep_dup
-      alias_method :deep_dup, :dup
-
       def marshal_dump
         result = [
           name,

activemodel/lib/active_model/type/string.rb

@@ -29,10 +29,6 @@ def to_immutable_string
         )
       end
 
-      def mutable? # :nodoc:
-        true
-      end
-
       private
         def cast_value(value)
           case value