webrick: add the ability to override res, req creation

So that a customized HTTPServer subclass can use it's own
Request/Response classes.

To apply the override, make a subclass of WEBrick::HTTPServer
and override the
`create_request_and_response(with_webrick_config)` method. The
method should return an Array of [request, response].

To check whether the Server supports this method (i.e. when
using older versions of WEBrick when needing this
functionality), you can ask the server if it responds to the
method

    server.respond_to?(:create_request_and_response)

This is backportable.

[ruby-core:69604] [Feature #11266]

From: Julik Tarkhanov <me@julik.nl>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66452 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/webrick/httpserver.rb

@@ -68,8 +68,8 @@ module WEBrick
 
     def run(sock)
       while true
-        res = HTTPResponse.new(@config)
-        req = HTTPRequest.new(@config)
+        req = create_request(@config)
+        res = create_response(@config)
         server = self
         begin
           timeout = @config[:RequestTimeout]
@@ -224,6 +224,20 @@ module WEBrick
       }
     end
 
+    ##
+    # Creates the HTTPRequest used when handling the HTTP
+    # request. Can be overridden by subclasses.
+    def create_request(with_webrick_config)
+      HTTPRequest.new(with_webrick_config)
+    end
+
+    ##
+    # Creates the HTTPResponse used when handling the HTTP
+    # request. Can be overridden by subclasses.
+    def create_response(with_webrick_config)
+      HTTPResponse.new(with_webrick_config)
+    end
+
     ##
     # Mount table for the path a servlet is mounted on in the directory space
     # of the server.  Users of WEBrick can only access this indirectly via