Added digest_class configuration option

[saghaulor]

• MD5 is not permitted in FIPS mode, nor is the Digest class. When operating with FIPS mode, only OpenSSL::Digest::SHAxxx is permitted.
  -Closes Dalli uses MD5 which is not permitted in FIPS mode #723

[petergoldstein]

@saghaulor So a couple of pieces of feedback:

1. This change needs one or more corresponding specs
2. We should not just allow the passing in of a random class, especially since it may or may not implement the hexdigest method. At a minimum there should be duck type checking. Alternatively, we can specify a list of acceptable digest classes and make that configurable with a string value.

Happy to see either option on #2.

[saghaulor]

@petergoldstein thanks for the great feedback. I'll add some more stringent checks on the digest_class option, and add some accompanying tests.

[saghaulor]

@petergoldstein I believe this PR is ready to merge if you agree with the changes. Please let me know if there is anything else you would like me to do regarding this PR.

[petergoldstein]

@saghaulor This looks good, thanks. Will merge. Not likely to get to a release immediately, but I'll try to do it within the next week or so.