Non-blocking socket not handled correctly.

https://github.com/ruby/openssl/blob/94fb921540fe49d3842b0ae562efbfea62a42901/ext/openssl/ossl_ssl.c#L2124

According to the documentation, SSL_shutdown can:

If the underlying BIO is nonblocking, SSL_shutdown() will also return when the underlying BIO could not satisfy the needs of SSL_shutdown() to continue the handshake. In this case a call to SSL_get_error() with the return value of SSL_shutdown() will yield SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE. The calling process then must repeat the call after taking appropriate action to satisfy the needs of SSL_shutdown(). The action depends on the underlying BIO. When using a nonblocking socket, nothing is to be done, but select() can be used to check for the required condition. When using a buffering BIO, like a BIO pair, data must be written into or retrieved out of the BIO before being able to continue.

I don't know if we need to explicitly handle this when in non-blocking mode or not, but I'd assume so until otherwise confirmed/checked.

Start with ext/openssl/ossl_ssl.c around line 2124 and compare the SSL_shutdown behavior there with the quoted OpenSSL documentation for non-blocking BIOs. Determine how SSL_ERROR_WANT_READ and SSL_ERROR_WANT_WRITE should be handled in non-blocking mode; done means the shutdown path correctly supports the documented behavior.