Custom WEBrick FileHandler for stripping .html extension

This first performs the normal static file check at the exact
location. If no file is found, attempt the check again with an
".html" suffix.

See the following for base class search_file implementation:

https://github.com/ruby/ruby/blob/4607f95/lib/webrick/httpservlet/filehandler.rb#L363-L383

lib/jekyll/commands/serve.rb

@@ -40,7 +40,7 @@ module Jekyll
 
           s.mount(
             options['baseurl'],
-            WEBrick::HTTPServlet::FileHandler,
+            FileHandler,
             destination,
             file_handler_options
           )
@@ -131,6 +131,19 @@ module Jekyll
 
       end
 
+      # Custom WEBrick FileHandler servlet for serving "/file.html" at "/file"
+      # when no exact match is found. This mirrors the behavior of GitHub Pages
+      # and many static web server configs.
+      require 'webrick'
+      class FileHandler < ::WEBrick::HTTPServlet::FileHandler
+        def search_file(req, res, basename)
+          if file = super
+            file
+          else
+            super(req, res, "#{basename}.html")
+          end
+        end
+      end
     end
   end
 end