Rails and GWT (HTTP request setHeader problems)

Hello
I have a server in rails and the client side in GWT…I have problem
with setting the header of a request in GWT i do something like this:

RequestBuilder builder = new RequestBuilder(RequestBuilder.GET,
http://localhost:3000/securities/"+security.getId()+"/market_overview”);
builder.setHeader(“HTTP_IF_MODIFIED_SINCE”, “Mon Oct 15 15:38:35
+0300 2007”);

and on RUBY SERVER i do:

minTime = Time.rfc2822(request.env[‘HTTP_IF_MODIFIED_SINCE’]) rescue nil
if (minTime and @market_overview.updated_at<= minTime)
render_text ‘’, ‘304 Not Modified’
else
respond_to do |format|
#inti securityURL for this MO
format.xml { render :xml => (@market_overview.to_xml
:skip_types => true)}
end
end

request.env[‘HTTP_IF_MODIFIED_SINCE’] is nil…I don’t know where I do
wrong …on the client side or on the server side…Please help me if
you can…Thanks!