Remove Mime::MSGPACK

Accessing to mime types through constants was removed since Rails 5.2.
rails/rails@2a3718f

Author: a2ikm

lib/msgpack_rails.rb

@@ -36,7 +36,7 @@ class Rails < ::Rails::Engine
         ::Mime::Type.register "application/msgpack", :msgpack
 
         ::ActionController::Renderers.add :msgpack do |data, options|
-          self.content_type = Mime::MSGPACK
+          self.content_type = Mime[:msgpack]
           self.response_body = data.is_a?(String) ? data : data.to_msgpack(options)
         end
       end