[authentication] OpenSSL::Digest::Digest is deprecated.

Fixes output of "Digest::Digest is deprecated; use Digest".

Deprecated all the way back to 1.8.7-p374:
https://github.com/ruby/ruby/blob/v1_8_7_374/ext/openssl/lib/openssl/digest.rb#L51-L57
introduced in 1.8.6-p5000
https://github.com/ruby/ruby/commit/00be63fca5e636383207c8916cce8c61b2059985