fix bug where proxying only works the first time

Author: Stephen Dawson-Haggerty

lib/rack/reverse_proxy.rb

@@ -152,7 +152,7 @@ def match?(path)
     end
 
     def get_uri(path,env)
-      _url=(url.respond_to?(:call) ? url.call(env) : url)
+      _url=(url.respond_to?(:call) ? url.call(env) : url.clone)
       if _url =~/\$\d/
         match_path(path).to_a.each_with_index { |m, i| _url.gsub!("$#{i.to_s}", m) }
         URI(_url)