ssl: initialize verify_mode and verify_hostname with default values

[rhenium]

test/openssl/test_ssl: revise a test case for client_cert_cb

The current test_client_auth_public_key test case checks that supplying
a PKey containing only public components through client_cert_cb will
cause handshake to fail. While this is a correct behavior as a whole,
the assertions are misleading in the sense that giving a public key is
causing the failure. Actually, the handshake fails because a client
certificate is not supplied at all, as a result of ArgumentError that is
silently ignored.

Rename the test case to test_client_cert_cb_ignore_error and simplify it
to clarify what it is testing.

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test/openssl/test_ssl: revise verify_mode test cases

Add explicit test cases for the behaviors with different verify_mode.
If we made a bug in verify_mode, we would notice it by failures of other
test cases, but there were no dedicated test cases for verify_mode.

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ssl: initialize verify_mode and verify_hostname with default values

SSLContext's verify_mode expects an SSL_VERIFY_* constant (an integer)
and verify_hostname expects either true or false. However, they are set
to nil after calling OpenSSL::SSL::SSLContext.new, which is surprising.

Set a proper value to them by default: verify_mode is set to
OpenSSL::SSL::VERIFY_NONE and verify_hostname is set to false by
default.

Note that this does not change the default behavior. The certificate
verification was never performed unless verify_mode is set to
OpenSSL::SSL::VERIFY_PEER by a user. The same applies to
verify_hostname.