Commit ad654793 will cause "Method Not Allow" in preflight response
when enable CORS plugin.

The root cause is that CORS plugin didn't generate http output after applied
commit ad654793, so the value of `ctx.ResponseWriter.Started` will be keep
`false`, and then later filter chains will be go on to run when CORS filter
finished.

This path will both fix "Method Not Allow" and the original bug
"multiple response.WriteHeader calls".
Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>