Use a ruby_bug guard since this bug fix will likely be backported

* And it should be tested on current versions of other Ruby implementations.

Author: eregon

spec/ruby/core/unboundmethod/equal_value_spec.rb

@@ -97,7 +97,7 @@
     (@includer == @includee).should == true
   end
 
-  ruby_version_is "4.1" do
+  ruby_bug "#21873", ""..."4.1" do
     it "returns true if same method is present in an object through module inclusion" do
       (@mixin == @includer_base).should == true
       (@includer_base == @mixin).should == true