Add way for controllers with different models to use the only parameter

The /emails endpoint was passing in the "Email" model because that's
how the emails controller classifies. This was to fix that, and to
allow any other such cases in the future.

app/controllers/application_controller.rb

@@ -35,8 +35,10 @@ class ApplicationController < ActionController::Base
       return
     end
 
+    model = options[0]&.delete(:model)
     if subject.respond_to?(:includes) && (request.format.json? || request.format.xml?)
-      associations = ParameterBuilder.includes_parameters(params[:only], model_name)
+      model ||= model_name
+      associations = ParameterBuilder.includes_parameters(params[:only], model)
       subject = subject.includes(associations)
     end