fix typo in spec name

Author: jjb

spec/rack/reverse_proxy_spec.rb

@@ -322,7 +322,7 @@ def app
       end
     end
 
-    describe "with preserve response host turned on" do
+    describe "with replace response host turned on" do
       def app
         Rack::ReverseProxy.new(dummy_app) do
           reverse_proxy "/test", "http://example.com/", :replace_response_host => true