Make ne_ssl_trust_default_ca a noop for non-SSL sessions

[notroj]

Make ne_ssl_trust_default_ca a noop for non-SSL sessions, like ne_ssl_trust_cert.

* src/ne_gnutls.c (ne_ssl_trust_default_ca),
  src/ne_openssl.c (ne_ssl_trust_default_ca): Noop for non-SSL session.

* test/ssl.c (nonssl_trust): Test that ne_ssl_trust_default_ca() is a 
  noop for non-SSL sessions.