Fix shared test digest_instance_update

Use the provided method instead. This fixes the `Digest::Instance#<<`
spec, it now does what it says it does.

library/digest/instance/shared/update.rb

@@ -3,6 +3,6 @@
     c = Class.new do
       include Digest::Instance
     end
-    -> { c.new.update("test") }.should raise_error(RuntimeError)
+    -> { c.new.send(@method, "test") }.should raise_error(RuntimeError)
   end
 end