call do_XXX which corespond with request method.
(WEBrick::HTTPProxyServer#do_CONNECT,do_GET,do_POST,do_HEAD): added.
* test/webrick/test_httpproxy.rb: add test for WEBrick::HTTPProxyServer.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@14816 b2dd03c8-39d4-4d8f-98ff-823fe69b080e