Fix set_the_flash_matcher for Rails 4

The "flashes" instance variable that lives within FlashHash has changed
in Rails 4. I have added a shim to get the proper name.

Additionally, the set_the_flash matcher will dup the flash hash for the
controller on which it operates. As #dup does a shallow copy we need to
make sure to also copy the instance variables within the flash hash when
we do this. The Rails 4 version of FlashHash has an extra @discard
variable unlike the Rails 3 version so we make sure to copy that.

lib/shoulda/matchers/action_controller/set_the_flash_matcher.rb

@@ -90,20 +90,34 @@ def flash_values
         end
 
         def flash
-          if @flash
-            @flash
-          else
-            @flash = @controller.flash.dup
-            used = @controller.flash.instance_variable_get(:@used).dup
-            @flash.instance_variable_set(:@used, used)
-            sweep_flash_if_necessary
-            @flash
+          @flash ||= copy_of_flash_from_controller
+        end
+
+        def copy_of_flash_from_controller
+          @controller.flash.dup.tap do |flash|
+            copy_flashes(@controller.flash, flash)
+            copy_discard_if_necessary(@controller.flash, flash)
+            sweep_flash_if_necessary(flash)
+          end
+        end
+
+        def copy_flashes(original_flash, new_flash)
+          flashes_ivar = Shoulda::Matchers::RailsShim.flashes_ivar
+          flashes = original_flash.instance_variable_get(flashes_ivar).dup
+          new_flash.instance_variable_set(flashes_ivar, flashes)
+        end
+
+        def copy_discard_if_necessary(original_flash, new_flash)
+          discard_ivar = :@discard
+          if original_flash.instance_variable_defined?(discard_ivar)
+            discard = original_flash.instance_variable_get(discard_ivar).dup
+            new_flash.instance_variable_set(discard_ivar, discard)
           end
         end
 
-        def sweep_flash_if_necessary
+        def sweep_flash_if_necessary(flash)
           unless @options[:now]
-            @flash.sweep
+            flash.sweep
           end
         end
 

lib/shoulda/matchers/rails_shim.rb

@@ -10,7 +10,13 @@ def self.layouts_ivar
         end
       end
 
-      private
+      def self.flashes_ivar
+        if rails_major_version >= 4
+          :@flashes
+        else
+          :@used
+        end
+      end
 
       def self.rails_major_version
         Rails::VERSION::MAJOR