Make Digest() thread-safe.

* ext/digest/lib/digest.rb (Digest()): This function should now be
  thread-safe.  If you have a problem with regard to on-demand
  loading under a multi-threaded environment, preload "digest/*"
  modules on boot or use this method instead of directly
  referencing Digest::*. [Bug #9494]
  cf. https://github.com/aws/aws-sdk-ruby/issues/525

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48213 b2dd03c8-39d4-4d8f-98ff-823fe69b080e