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/

CHANGELOG.md

@@ -7,6 +7,7 @@
 #### Fixes
 
 * Your contribution here.
+* [#2104](https://github.com/ruby-grape/grape/pull/2104): Fix Ruby 2.7 keyword deprecation warning - [@stanhu](https://github.com/stanhu).
 * [#2103](https://github.com/ruby-grape/grape/pull/2103): Ensure complete declared params structure is present - [@tlconnor](https://github.com/tlconnor).
 * [#2099](https://github.com/ruby-grape/grape/pull/2099): Added truffleruby to Travis-CI - [@gogainda](https://github.com/gogainda).
 * [#2089](https://github.com/ruby-grape/grape/pull/2089): Specify order of mounting Grape with Rack::Cascade in README - [@jonmchan](https://github.com/jonmchan).

lib/grape/dsl/inside_route.rb

@@ -422,7 +422,7 @@ 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