This is same issue described in https://github.com/h2o/h2o/issues/268.
That is if SSL object has decrypted data buffered inside it, and
application does not read it for some reason (e.g., rate limit), we
have to check the existence of data using SSL_pending.  This is
because buffered data inside SSL is not notified by io watcher.  It is
obvious, but we totally missed it.

nghttpx code normally reads everything until SSL_read returns error
(want-read).  But if rate limit is involved, we stop reading early.
Also in HTTP/1 code, while processing one request, we just read until
buffer is filled up.  In these cases, we may suffer from this problem.

This commit fixes this problem, by performing SSL_pending() and if it
has buffered data and read io watcher is enabled, we feed event using
ev_feed_event().