Don't try to destoy the given string via method argument

Or it would raise if the argument was frozen.

And even with this change, it would still reduce String allocations together with 9276ea89d2
because `escape` should be `true` in most cases

actionview/lib/action_view/helpers/tag_helper.rb

@@ -88,7 +88,7 @@ module ActionView
           if value.is_a?(Array)
             value = escape ? safe_join(value, " ".freeze) : value.join(" ".freeze)
           else
-            value = escape ? ERB::Util.unwrapped_html_escape(value) : value.to_s
+            value = escape ? ERB::Util.unwrapped_html_escape(value) : value.to_s.dup
           end
           value.gsub!('"'.freeze, '"'.freeze)
           %(#{key}="#{value}")