always use buffer, if connection is buffered, this fixes OpenSSL "bad write retry" error, when *) nginx passed a single buf greater than our buffer (say 32K) to OpenSSL, *) OpenSSL returns SSL_ERROR_WANT_WRITE, *) after some time nginx has to send a new data, *) so there are at least two bufs nginx does pass them directly to OpenSSL, *) but copies the first buf part to buffer, and sends the buffer to OpenSSL. *) because the data length is lesser than it was in previous SSL_write(): 16K < 32K, OpenSSL returns SSL_R_BAD_WRITE_RETRY.