Fix tinymce feature spec

This feature spec failed with Rails 6.1 as it was defaulting to example.com instead of 127.0.0.1.

Setting `asset_host` inside the test is more explicit as it was before and reflects what an actual app does.

spec/features/admin/tinymce_feature_spec.rb

@@ -16,13 +16,13 @@
 
   context "with asset host" do
     before do
-      expect(ActionController::Base.config).to receive(:asset_host_set?).and_return(true)
+      ActionController::Base.config.asset_host = "myhost.com"
     end
 
     it "base path should be set to tinymce asset folder" do
       visit admin_dashboard_path
       expect(page).to have_content(
-        "var tinyMCEPreInit = { base: 'http://127.0.0.1/assets/tinymce', suffix: '.min' };",
+        "var tinyMCEPreInit = { base: 'http://myhost.com/assets/tinymce', suffix: '.min' };",
       )
     end
   end