Fix deprecation warnings by switching text to plain

Fixes deprecation warning to swicth from using `render text:` to `render
plain:`.

```
`render :text` is deprecated because it does not actually render a
`text/plain` response. Switch to `render plain: 'plain text'` to render
as `text/plain`
```

test/webservice_test.rb

@@ -4,9 +4,9 @@ class WebServiceTest < ActionDispatch::IntegrationTest
   class TestController < ActionController::Base
     def assign_parameters
       if params[:full]
-        render :text => dump_params_keys
+        render plain: dump_params_keys
       else
-        render :text => (params.keys - ['controller', 'action']).sort.join(", ")
+        render plain: (params.keys - ['controller', 'action']).sort.join(", ")
       end
     end