Use OpenSSL::Digest instead of digest gem

[hsbt]

digest has been removed at #38.

On another note, openssl provide digest methods same as digest. We should use OpenSSL::Digest for reducing dependencies.

[tmtm]

I would prefer Digest to be optional rather than requiring OpenSSL.

[hsbt]

After releasing net-smtp gem, the users already have openssl because they access rubygems.org via https. There is no strong reason avoid openssl dependency.

Can you describe your motivation?

[tmtm]

rubygems.org is a very large gem distribution site, but I think there are ways to install gems other than downloading them directly from rubygems.org.
For example, copy the gem file downloaded from rubygems.org and install it in a Ruby environment that does not support OpenSSL.
Ruby works without OpenSSL, so it is better to be able to use net/smtp even in environments where TLS communication is not required for SMTP.

[rhenium]

Note that openssl still depends on digest. OpenSSL::Digest is an implementation of Digest::Class.

Generally speaking, OpenSSL::Digest is useful if the performance is important or an algorithm not supported by digest is required. IMO the current use of Digest::MD5 here is fine.

[hsbt]

Ruby works without OpenSSL

I'm against this use-case. We should avoid to use network access without openssl today.

But I have no strong opinion this change. I'll close this.

[hsbt]

@rhenium Thanks for explanation. I misunderstood the internal of OpenSSL::Digest independant from digest.