Fixed HTML escaping after call to html_tag

The old value of _haml_concat_raw was not being set correctly, hence
calls to haml_concat after a call to haml_tag would result in the output
of haml_concat being appeneded unescaped.

Resolves #718

lib/haml/helpers/action_view_extensions.rb

@@ -45,8 +45,8 @@ def page_class
       # @yield A block in which all input to `#haml_concat` is treated as raw.
       # @see Haml::Util#rails_xss_safe?
       def with_raw_haml_concat
+        old = instance_variable_defined?('@_haml_concat_raw') ? @_haml_concat_raw : false
         @_haml_concat_raw = true
-        old = @_haml_concat_raw
         yield
       ensure
         @_haml_concat_raw = old

test/helper_test.rb

@@ -14,6 +14,16 @@ module Haml::Helpers
   def something_that_uses_haml_concat
     haml_concat('foo').to_s
   end
+
+  def render_something_with_haml_concat
+    haml_concat "<p>"
+  end
+
+  def render_something_with_haml_tag_and_concat
+    haml_tag 'p' do
+      haml_concat '<foo>'
+    end
+  end
 end
 
 class HelperTest < MiniTest::Unit::TestCase
@@ -42,6 +52,15 @@ def render(text, options = {})
     super
   end
 
+  def test_rendering_with_escapes
+    output = render(<<-HAML, :action_view)
+- render_something_with_haml_concat
+- render_something_with_haml_tag_and_concat
+- render_something_with_haml_concat
+HAML
+    assert_equal("&lt;p&gt;\n<p>\n  <foo>\n</p>\n&lt;p&gt;\n", output)
+  end
+
   def test_flatten
     assert_equal("FooBar", Haml::Helpers.flatten("FooBar"))