Add handler existence check to fallback to apache
Add handler existence check to fallback to apache

Recent change broke apache fallback compatibility because when all supporting
versions of handshake result in failure, we respond 400 bad request as http
response.

This change support apache fallback again on URLs on which pywebsocket doesn't
have any handler scripts.

Review URL: http://codereview.appspot.com/5302050

git-svn-id: a751b5b3dcfba0ee4592a85c40d2fdd063ca0d53@583 4ff78f4a-9131-11de-b045-6380ec9940d4

XiaonuoGantan/pywebsocket,XiaonuoGantan/pywebsocket