Fix Ruby 2.7 deprecation warning

This fixes the warning:

```
ruby/2.7.0/gems/grape-1.4.0/lib/grape/dsl/inside_route.rb: warning:
Using the last argument as keyword parameters is deprecated; maybe **
should be added to the call
```

More details:
https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/

lib/grape/dsl/inside_route.rb

@@ -435,7 +435,7 @@ def entity_class_for_obj(object, options)
       def entity_representation_for(entity_class, object, options)
         embeds = { env: env }
         embeds[:version] = env[Grape::Env::API_VERSION] if env[Grape::Env::API_VERSION]
-        entity_class.represent(object, embeds.merge(options))
+        entity_class.represent(object, **embeds.merge(options))
       end
     end
   end