Raise NoMethodError in ActiveModel::Type::Value#as_json method.

Right now since we have instance variable called `itself_if_serialize_cast_value_compatible`
assigned to self when we run `as_json` we get stack too deep error because `as_json` calls
`as_json` on every instance variable. And since `@itself_if_serialize_cast_value_compatible` references
to `self` we run into recursion.

And before that we were returning unpredictable data from `as_json` method so it's better to let it to throw
an error and let user know that Value class is not supposed to be converted to json.

activemodel/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Raise `NoMethodError` in `ActiveModel::Type::Value#as_json` to avoid unpredictable
+    results.
+
+    *Vasiliy Ermolovich*
+
 *   Custom attribute types that inherit from Active Model built-in types and do
     not override the `serialize` method will now benefit from an optimization
     when serializing attribute values for the database.

activemodel/lib/active_model/type/value.rb

@@ -135,6 +135,10 @@ def immutable_value(value) # :nodoc:
         value
       end
 
+      def as_json(*)
+        raise NoMethodError
+      end
+
       private
         # Convenience method for types which do not need separate type casting
         # behavior for user and database inputs. Called by Value#cast for

activemodel/test/cases/type/value_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "cases/helper"
+require "active_support/core_ext/object/json"
 
 module ActiveModel
   module Type
@@ -10,6 +11,12 @@ def test_type_equality
         assert_not_equal Type::Value.new, Type::Integer.new
         assert_not_equal Type::Value.new(precision: 1), Type::Value.new(precision: 2)
       end
+
+      def test_as_json_not_defined
+        assert_raises NoMethodError do
+          Type::Value.new.as_json
+        end
+      end
     end
   end
 end