Add test/doc for :if/:except in skip_before_action

The new test/docs further explain the conflicts that can happen when
mixing `:if`/`:unless` options with `:only`/`:except` options in
`skip_before_action`.

The gist is that "positive" filters always have priority over negative
ones.

The previous commit already showed that `:only` has priority over `:if`.

This commit shows that `:if` has priority over `:except`.

For instance, the following snippets are equivalent:

```ruby
skip_before_action :some_callback, if: -> { condition }, except: action
```

```ruby
skip_before_action :some_callback, if: -> { condition }
```

actionpack/lib/abstract_controller/callbacks.rb

@@ -33,6 +33,11 @@ module AbstractController
       #
       #   only: :index, if: -> { true } # the :if option will be ignored.
       #
+      # Note that <tt>:if</tt> has priority over <tt>:except</tt> in case they
+      # are used together.
+      #
+      #   except: :index, if: -> { true } # the :except option will be ignored.
+      #
       # ==== Options
       # * <tt>only</tt>   - The callback should be run only for this action
       # * <tt>except</tt>  - The callback should be run for all actions except this action

actionpack/test/controller/filters_test.rb

@@ -225,7 +225,7 @@ class FilterTest < ActionController::TestCase
     skip_before_action :clean_up_tmp, if: -> { true }
   end
 
-  class SkipFilterUsingOnlyAndConditional < ConditionalFilterController
+  class SkipFilterUsingOnlyAndIf < ConditionalFilterController
     before_action :clean_up_tmp
     before_action :ensure_login
 
@@ -237,6 +237,18 @@ class FilterTest < ActionController::TestCase
     end
   end
 
+  class SkipFilterUsingIfAndExcept < ConditionalFilterController
+    before_action :clean_up_tmp
+    before_action :ensure_login
+
+    skip_before_action :ensure_login, if: -> { false }, except: :login
+    skip_before_action :clean_up_tmp, if: -> { true }, except: :login
+
+    def login
+      render text: 'ok'
+    end
+  end
+
   class ClassController < ConditionalFilterController
     before_action ConditionalClassFilter
   end
@@ -609,10 +621,15 @@ class FilterTest < ActionController::TestCase
   end
 
   def test_if_is_ignored_when_used_with_only
-    test_process(SkipFilterUsingOnlyAndConditional, 'login')
+    test_process(SkipFilterUsingOnlyAndIf, 'login')
     assert_nil assigns['ran_filter']
   end
 
+  def test_except_is_ignored_when_used_with_if
+    test_process(SkipFilterUsingIfAndExcept, 'login')
+    assert_equal %w(ensure_login), assigns["ran_filter"]
+  end
+
   def test_skipping_class_actions
     test_process(ClassController)
     assert_equal true, assigns["ran_class_action"]