Enable option for index.html to be a directories default (#640)

spec/static_file_handler_spec.cr

@@ -23,6 +23,15 @@ describe Kemal::StaticFileHandler do
     response.body.should eq(File.read("#{__DIR__}/static/dir/test.txt"))
   end
 
+  it "should serve the 'index.html' file when a directory is requested and index serving is enabled" do
+    serve_static({"dir_index" => true})
+    response = handle HTTP::Request.new("GET", "/dir/")
+    response.status_code.should eq(200)
+    response.headers["Content-Type"].should eq "text/html"
+    response.headers["Etag"].should contain "W/\""
+    response.body.should eq(File.read("#{__DIR__}/static/dir/index.html"))
+  end
+
   it "should respond with 304 if file has not changed" do
     response = handle HTTP::Request.new("GET", "/dir/test.txt")
     response.status_code.should eq(200)

src/kemal/config.cr

@@ -31,7 +31,7 @@ module Kemal
       @host_binding = "0.0.0.0"
       @port = 3000
       @env = ENV["KEMAL_ENV"]? || "development"
-      @serve_static = {"dir_listing" => false, "gzip" => true}
+      @serve_static = {"dir_listing" => false, "gzip" => true, "dir_index" => false}
       @public_folder = "./public"
       @logging = true
       @logger = nil

src/kemal/static_file_handler.cr

@@ -36,16 +36,27 @@ module Kemal
                     end
 
       file_path = File.join(@public_dir, expanded_path)
-      is_dir = Dir.exists? file_path
+      is_dir = Dir.exists?(file_path)
 
       if request_path != expanded_path
         redirect_to context, expanded_path
       elsif is_dir && !is_dir_path
         redirect_to context, expanded_path + '/'
       end
 
-      if Dir.exists?(file_path)
-        if config.is_a?(Hash) && config["dir_listing"] == true
+      if is_dir
+        if config.is_a?(Hash) && config.fetch("dir_index", false) && File.exists?(File.join(file_path, "index.html"))
+          file_path = File.join(@public_dir, expanded_path, "index.html")
+
+          last_modified = modification_time(file_path)
+          add_cache_headers(context.response.headers, last_modified)
+
+          if cache_request?(context, last_modified)
+            context.response.status_code = 304
+            return
+          end
+          send_file(context, file_path)
+        elsif config.is_a?(Hash) && config.fetch("dir_listing", false)
           context.response.content_type = "text/html"
           directory_listing(context.response, request_path, file_path)
         else