flip conditional to use if/else instead of unless/else

Use if/else instead of unless/else so conditional reads better.

activesupport/lib/active_support/callbacks.rb

@@ -297,14 +297,14 @@ def self.halting(next_callback, user_callback)
             target = env.target
             value  = env.value
 
-            unless env.halted
+            if env.halted
+              next_callback.call env
+            else
               user_callback.call(target, value) {
                 env = next_callback.call env
                 env.value
               }
               env
-            else
-              next_callback.call env
             end
           }
         end