Add tests for Fields::Types::Enum#enum when no bindings provided

A lack of tests for these code paths led to a regression for legacy enums in Rails Admin 3.0 (#3651).

This tests the code path using `abstract_model` that was fixed by @fuegas in the previous commit.

Author: robotfelix

spec/rails_admin/config/fields/types/enum_spec.rb

@@ -63,10 +63,15 @@ def color_list
       Team.send(:remove_method, :color_list)
     end
 
-    it 'allows configuration' do
+    it 'allows configuration by enum_method when bindings provide object' do
       is_expected.to be_a(RailsAdmin::Config::Fields::Types::Enum)
       expect(subject.with(object: Team.new).enum).to eq %w[blue green red]
     end
+
+    it 'allows configuration by enum_method when no bindings provided' do
+      is_expected.to be_a(RailsAdmin::Config::Fields::Types::Enum)
+      expect(subject.enum).to eq %w[blue green red]
+    end
   end
 
   describe 'the enum class method' do
@@ -87,10 +92,15 @@ def color_list
       Team.instance_eval { undef :color_list }
     end
 
-    it 'allows configuration' do
+    it 'allows configuration by enum_method when bindings provide object' do
       is_expected.to be_a(RailsAdmin::Config::Fields::Types::Enum)
       expect(subject.with(object: Team.new).enum).to eq %w[blue green red]
     end
+
+    it 'allows configuration by enum_method when no bindings provided' do
+      is_expected.to be_a(RailsAdmin::Config::Fields::Types::Enum)
+      expect(subject.enum).to eq %w[blue green red]
+    end
   end
 
   describe 'when overriding enum configuration' do