db._authOrThrow should prioritize user specified authenticationMechanism instead of server's mechanisms

In SERVER-32977, we prioritized the server's SASL mechanism priority over the client's choice. If the server supports Kerberos via GSSAPI, and PLAIN via LDAP authentication, the shell will try to connect using GSSAPI instead of PLAIN.

This occurs because in DB.prototype._authOrThrow, we never pass mechanism, take the following if, and this ignores the user's mechanism choice which sits in this._defaultAuthenticationMechanism.

        if (params.mechanism === undefined) {

            params.mechanism = this._getDefaultAuthenticationMechanism(params.user, this.getName());

        }