Fix duplicated 'back_url' IDs

Various hidden input fields are used to hold a URL to send the user back
to the correct page after submitting a form, however, they all use the
same ID which isn't allowed in HTML. Passing in 'id' as nil stops the
'hidden_field_tag' from adding an ID attribute to the tag.

Contributed by Andrew Smith.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@9250 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/views/issues/_list.html.erb

@@ -1,5 +1,5 @@
 <% form_tag({}) do -%>
-<%= hidden_field_tag 'back_url', url_for(params) %>
+<%= hidden_field_tag 'back_url', url_for(params), :id => nil %>
 <div class="autoscroll">
 <table class="list issues">
     <thead><tr>