Fix ActiveStorage use-case and add test case. Fixes #1464.

lib/rack/files.rb

@@ -22,7 +22,7 @@ class Files
     attr_reader :root
 
     def initialize(root, headers = {}, default_mime = 'text/plain')
-      @root = ::File.expand_path root
+      @root = (::File.expand_path(root) if root)
       @headers = headers
       @default_mime = default_mime
       @head = Rack::Head.new(lambda { |env| get env })

test/spec_files.rb

@@ -12,6 +12,20 @@ def files(*args)
     Rack::Lint.new Rack::Files.new(*args)
   end
 
+  it "can be used without root" do
+    # https://github.com/rack/rack/issues/1464
+
+    app = Rack::Files.new(nil)
+
+    request = Rack::Request.new(
+      Rack::MockRequest.env_for("/cgi/test")
+    )
+
+    file_path = File.expand_path("cgi/test", __dir__)
+    status, headers, body = app.serving(request, file_path)
+    assert_equal 200, status
+  end
+
   it 'serves files with + in the file name' do
     Dir.mktmpdir do |dir|
       File.write File.join(dir, "you+me.txt"), "hello world"