otherwise RSA keys will use built-in RSA methods

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.