[refactor] Migrate from deprecated Rack::File to Rack::Files

See rack/rack@626272b

Author: chadlwilson

src/main/ruby/jruby/rack/error_app.rb

@@ -31,7 +31,7 @@ class ErrorApp
       attr_reader :root
 
       def initialize(root = nil)
-        if defined?(::Rack::File) && root.is_a?(::Rack::File)
+        if defined?(::Rack::Files) && root.is_a?(::Rack::Files)
           @root = root.root # backwards compatibility
         else
           @root = root.nil? ? JRuby::Rack.public_path : root

src/main/ruby/rack/adapter/rails.rb

@@ -11,7 +11,7 @@ class Rails
       def initialize(options={})
         @root   = options[:root]   || Dir.pwd
         @public = options[:public] || ::File.join(@root, "public")
-        @file_server = Rack::File.new(@public)
+        @file_server = Rack::Files.new(@public)
         if defined?(ActionController::Dispatcher.middleware)
           @dispatcher = ActionController::Dispatcher.new
         else