Fix serving files that clash with directories

Fix jekyll#6222, related to jekyll#3452

See base class search_index_file implementation:

https://github.com/ruby/webrick/blob/3fb6a011/lib/webrick/httpservlet/filehandler.rb#L356-L363

lib/jekyll/commands/serve/servlet.rb

@@ -22,7 +22,7 @@ def initialize(server, root, callbacks)
 
         def search_file(req, res, basename)
           # /file.* > /file/index.html > /file.html
-          super || super(req, res, "#{basename}.html")
+          super || super(req, res, ".html") || super(req, res, "#{basename}.html")
         end
 
         # rubocop:disable Style/MethodName