Avoid checking defined?(@html_safe) (rails#45620)

We should not need to check defined? here because we are only interested
in whether @html_safe is truthy or falsy.

We can  use an aliased attr_reader to make this even faster by skipping
both method dispatch.

Co-authored-by: Jean Boussier <[email protected]>

Author: jhawthorn

activesupport/lib/active_support/core_ext/string/output_safety.rb

@@ -261,9 +261,9 @@ def %(args)
       self.class.new(super(escaped_args))
     end
 
-    def html_safe?
-      defined?(@html_safe) && @html_safe
-    end
+    attr_reader :html_safe
+    alias_method :html_safe?, :html_safe
+    remove_method :html_safe
 
     def to_s
       self

activesupport/test/safe_buffer_test.rb

@@ -171,6 +171,13 @@ def test_titleize
     assert_not_predicate @buffer.dup, :html_safe?
   end
 
+  test "Can call html_safe on a safe buffer" do
+    @buffer = "hello".html_safe
+    extra_safe = @buffer.html_safe
+    assert_equal "hello", extra_safe
+    assert_predicate extra_safe, :html_safe?
+  end
+
   test "Should return safe buffer when added with another safe buffer" do
     clean = "<script>".html_safe
     result_buffer = @buffer + clean