Move escape_once logic to ERB::Util, where it belongs to

All the logic is based on the HTML_ESCAPE constant available in
ERB::Util, so it seems more logic to have the entire method there and
just delegate the helper to use it.

actionpack/lib/action_view/helpers/tag_helper.rb

@@ -118,7 +118,7 @@ def cdata_section(content)
       #   escape_once("<< Accept & Checkout")
       #   # => "<< Accept & Checkout"
       def escape_once(html)
-        html.to_s.gsub(/[\"><]|&(?!([a-zA-Z]+|(#\d+));)/) { |special| ERB::Util::HTML_ESCAPE[special] }
+        ERB::Util.html_escape_once(html)
       end
 
       private

actionpack/test/template/erb_util_test.rb

@@ -44,4 +44,18 @@ def test_rest_in_ascii
       assert_equal chr, html_escape(chr)
     end
   end
+
+  def test_html_escape_once
+    assert_equal '1 &lt; 2 &amp; 3', html_escape_once('1 < 2 &amp; 3')
+  end
+
+  def test_html_escape_once_returns_unsafe_strings_when_passed_unsafe_strings
+    value = html_escape_once('1 < 2 &amp; 3')
+    assert !value.html_safe?
+  end
+
+  def test_html_escape_once_returns_safe_strings_when_passed_safe_strings
+    value = html_escape_once('1 < 2 &amp; 3'.html_safe)
+    assert value.html_safe?
+  end
 end

activesupport/CHANGELOG.md

@@ -1,5 +1,7 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*    Add html_escape_once to ERB::Util, and delegate escape_once tag helper to it. *Carlos Antonio da Silva*
+
 *    Remove ActiveSupport::TestCase#pending method, use `skip` instead. *Carlos Antonio da Silva*
 
 *    Deprecates the compatibility method Module#local_constant_names,

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

@@ -33,6 +33,21 @@ def html_escape(s)
     singleton_class.send(:remove_method, :html_escape)
     module_function :html_escape
 
+    # Returns an escaped version of +html+ without affecting existing escaped entities.
+    #
+    # ==== Examples
+    #   html_escape_once("1 < 2 &amp; 3")
+    #   # => "1 &lt; 2 &amp; 3"
+    #
+    #   html_escape_once("&lt;&lt; Accept & Checkout")
+    #   # => "&lt;&lt; Accept &amp; Checkout"
+    def html_escape_once(s)
+      result = s.to_s.gsub(/[\"><]|&(?!([a-zA-Z]+|(#\d+));)/) { |special| HTML_ESCAPE[special] }
+      s.html_safe? ? result.html_safe : result
+    end
+
+    module_function :html_escape_once
+
     # A utility method for escaping HTML entities in JSON strings
     # using \uXXXX JavaScript escape sequences for string literals:
     #