remove notify_id reqiure validate
fix test
bug fix: The Alipay return_url calls do not include the sign parameter.
So only validate the sign parameter if it's included or if the request
includes the notify_id parameter too.
Merge pull request #4 from asaph/alipay_return_url_fix

bug fix: The Alipay return_url calls do not include the sign parameter.
utf-8 char fix