Avoid modification of string literal in sequel plugin spec

spec/sequel_plugin_spec.rb

@@ -26,7 +26,7 @@ def s.humanize; 'X' end
   end
 
   it "should have humanize handle objects that support #humanize" do
-    s = 'x_b_id'
+    s = 'x_b_id'.dup
     class << s
       undef :humanize if method_defined?(:humanize)
     end