digest.c: fix #== for non-string arguments

* ext/digest/digest.c (rb_digest_instance_equal):
  fix #== for non-string arguments. [ruby-core:62967] [Bug #9913]
* test/digest/test_digest.rb: add test for above.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46368 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-06-07

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46369 b2dd03c8-39d4-4d8f-98ff-823fe69b080e