Treat Middleware::Base#initialize options as Hash. (#2064)

Author: skarger

CHANGELOG.md

@@ -9,6 +9,7 @@
 #### Fixes
 
 * Your contribution here.
+* [#2064](https://github.com/ruby-grape/grape/pull/2064): Fix Ruby 2.7 deprecation warning in `Grape::Middleware::Base#initialize` - [@skarger](https://github.com/skarger).
 
 ### 1.3.3 (2020/05/23)
 

lib/grape/middleware/base.rb

@@ -14,9 +14,9 @@ class Base
 
       # @param [Rack Application] app The standard argument for a Rack middleware.
       # @param [Hash] options A hash of options, simply stored for use by subclasses.
-      def initialize(app, **options)
+      def initialize(app, options = {})
         @app = app
-        @options = default_options.merge(**options)
+        @options = default_options.merge(options)
         @app_response = nil
       end