Allow attribute's default to be configured but keeping its own type

```ruby
class Post < ActiveRecord::Base
  attribute :written_at, default: -> { Time.now.utc }
end

# Rails 6.0
Post.type_for_attribute(:written_at) # => #<Type::Value ... precision: nil, ...>

# Rails 6.1
Post.type_for_attribute(:written_at) # => #<Type::DateTime ... precision: 6, ...>
```

This is an alternative of #39797.

Context https://github.com/rails/rails/pull/39797#issuecomment-655191817.

If people intend to override the existing type on the attribute, usually
an overriding type is explicitly specified, so I agree that the current
behavior (drop existing type information if type is omitted) is
practically quite useless, and it is almost like a bug in that there is
no way to override just the default.

So I'd like to change the current behavior on existing attributes
without a deprecation as a fix.

Closes #39797.

See also https://github.com/kufu/activerecord-bitemporal/pull/57.

activerecord/CHANGELOG.md

+*   Allow attribute's default to be configured but keeping its own type.
+
+    ```ruby
+    class Post < ActiveRecord::Base
+      attribute :written_at, default: -> { Time.now.utc }
+    end
+
+    # Rails 6.0
+    Post.type_for_attribute(:written_at) # => #<Type::Value ... precision: nil, ...>
+
+    # Rails 6.1
+    Post.type_for_attribute(:written_at) # => #<Type::DateTime ... precision: 6, ...>
+    ```
+
+    *Ryuta Kamizono*
+
 *   Allow default to be configured for Enum.
 
     ```ruby

activerecord/lib/active_record/attributes.rb

@@ -253,7 +253,7 @@ def load_schema! # :nodoc:
           when Proc
             type = type[type_for_attribute(name)]
           else
-            type ||= Type::Value.new
+            type ||= type_for_attribute(name)
           end
 
           define_attribute(name, type, **options.slice(:default))

activerecord/test/cases/attributes_test.rb

@@ -56,6 +56,18 @@ class CustomPropertiesTest < ActiveRecord::TestCase
       assert_equal 255, UnoverloadedType.type_for_attribute("overloaded_string_with_limit").limit
     end
 
+    test "overloaded default but keeping its own type" do
+      klass = Class.new(UnoverloadedType) do
+        attribute :overloaded_string_with_limit, default: "the overloaded default"
+      end
+
+      assert_equal 255, UnoverloadedType.type_for_attribute("overloaded_string_with_limit").limit
+      assert_equal 255, klass.type_for_attribute("overloaded_string_with_limit").limit
+
+      assert_nil UnoverloadedType.new.overloaded_string_with_limit
+      assert_equal "the overloaded default", klass.new.overloaded_string_with_limit
+    end
+
     test "extra options are forwarded to the type caster constructor" do
       klass = Class.new(OverloadedType) do
         attribute :starts_at, :datetime, precision: 3, limit: 2, scale: 1