Added support for more secure HMAC algorithms

Until now only hmac-md5 and hmac-sha1 were supported.
This update adds support for hmac-sha2-256 and hmac-sha2-512 to support connections to servers that only accept SHA2 HMAC algorithms.
Additionally this may also increase security even when connecting to servers that still accept the less secure HMAC algorithms (if they also support and prefer the SHA2 HMAC algorithms).