Preserve empty PATH_INFO for mounts

Allow PATH_INFO to remain an empty string when a request is made for the exact prefix name of a mount, e.g. a request to "/settings" for this route:

```
mount ->(env) { [200, {}, [env["PATH_INFO"]] }, at: "/settings"
```

This ensures that the Rack environment can be used to reconstruct the same path as used for the original request (e.g. "/settings" instead of "/settings/" for the example above).

To ensure compatibility for routers used within mounts, consider an empty string to be equivalent to "/" for the purposes of route matching.

This allows a router to be mounted and have it root route be used when a request is made to the exact mount prefix without any trailing slash:

```
mounted_router = Hanami::Router.new do
  root to: ->(env) { [200, {}, ["Hello from root"]] }
end

mount mounted_router, at: "/settings"
```

In the above example, a request to /settings will still serve the route returning "Hello from root".

Author: timriley

lib/hanami/router.rb

@@ -642,7 +642,12 @@ def recognize(env, params = {}, options = {})
     # @since 2.0.0
     # @api private
     def fixed(env)
-      @fixed.dig(env[::Rack::REQUEST_METHOD], env[::Rack::PATH_INFO])
+      path_info = env[::Rack::PATH_INFO]
+      # Treat empty PATH_INFO as "/" for route matching. This allows root routes (defined as "/") to
+      # match the empty PATH_INFO that is set for requests to a mount without a trailing slash.
+      path_info = DEFAULT_PREFIX if path_info == EMPTY_STRING
+
+      @fixed.dig(env[::Rack::REQUEST_METHOD], path_info)
     end
 
     # @since 2.0.0

lib/hanami/router/mounted_path.rb

@@ -20,7 +20,6 @@ def endpoint_and_params(env)
           matched_path = match.to_s
           env[::Rack::SCRIPT_NAME] = env[::Rack::SCRIPT_NAME].to_s + matched_path
           env[::Rack::PATH_INFO] = env[::Rack::PATH_INFO].sub(matched_path, EMPTY_STRING)
-          env[::Rack::PATH_INFO] = DEFAULT_PREFIX if env[::Rack::PATH_INFO] == EMPTY_STRING
         end
 
         [@app, match.named_captures]

spec/integration/hanami/router/mount_spec.rb

@@ -98,4 +98,47 @@
       end
     end
   end
+
+  context "PATH_INFO handling" do
+    let(:router) do
+      root_endpoint = ->(env) {
+        path_info = env[Rack::PATH_INFO]
+        [200, rack_headers({"Content-Length" => path_info.bytesize.to_s}), [path_info]]
+      }
+      articles_endpoint = ->(_env) {
+        [200, rack_headers({"Content-Length" => "9"}), ["/articles"]]
+      }
+
+      mounted_router = Hanami::Router.new do
+        get "/", to: root_endpoint
+        get "/articles", to: articles_endpoint
+      end
+
+      Hanami::Router.new do
+        mount mounted_router, at: "/api"
+      end
+    end
+    let(:app) { Rack::MockRequest.new(router) }
+
+    it "preserves empty PATH_INFO and matches the root route when request matches mount point exactly" do
+      response = app.request("GET", "/api", lint: true)
+
+      expect(response.status).to eq(200)
+      expect(response.body).to eq("")
+    end
+
+    it "preserves slash PATH_INFO and matches the root route when request has trailing slash" do
+      response = app.request("GET", "/api/", lint: true)
+
+      expect(response.status).to eq(200)
+      expect(response.body).to eq("/")
+    end
+
+    it "passes PATH_INFO for sub-paths" do
+      response = app.request("GET", "/api/articles", lint: true)
+
+      expect(response.status).to eq(200)
+      expect(response.body).to eq("/articles")
+    end
+  end
 end

spec/support/fixtures.rb

@@ -211,7 +211,7 @@ module Api
   class App
     def call(env)
       case env["PATH_INFO"]
-      when "/"
+      when ""
         [200, {}, ["home"]]
       when "/articles"
         [200, {}, ["articles"]]

spec/unit/hanami/router/mounted_path_spec.rb

@@ -58,12 +58,12 @@
         expect(env[Rack::PATH_INFO]).to eq("/orders")
       end
 
-      it "uses a slash for the PATH_INFO if it would otherwise be empty" do
+      it "uses an empty string for the PATH_INFO when the request matches the mount point exactly" do
         env.merge!(Rack::PATH_INFO => "/api")
 
         subject.endpoint_and_params(env)
 
-        expect(env[Rack::PATH_INFO]).to eq("/")
+        expect(env[Rack::PATH_INFO]).to eq("")
       end
     end