Make FlashHash#key? work with symbol and string

Closes #17586

actionpack/lib/action_dispatch/middleware/flash.rb

@@ -129,7 +129,7 @@ def keys
       end
 
       def key?(name)
-        @flashes.key? name
+        @flashes.key? name.to_s
       end
 
       def delete(key)

actionpack/test/controller/flash_hash_test.rb

@@ -29,6 +29,15 @@ def test_update
       assert_equal 'world', @hash['hello']
     end
 
+    def test_key
+      @hash['foo'] = 'bar'
+
+      assert @hash.key?('foo')
+      assert @hash.key?(:foo)
+      assert_not @hash.key?('bar')
+      assert_not @hash.key?(:bar)
+    end
+
     def test_delete
       @hash['foo'] = 'bar'
       @hash.delete 'foo'