class ActionDispatch::Routing::RoutesProxy

def merge_script_names(previous_script_name, new_script_name)

script name resolver for the mount point dependent part.
about since it depends on the specific request, but use our
context via ENV["SCRIPT_NAME"], which `mount` doesn't know
Keeps the part of the script name provided by the global 
def merge_script_names(previous_script_name, new_script_name)
  return new_script_name unless previous_script_name
  resolved_parts = new_script_name.count("/")
  previous_parts = previous_script_name.count("/")
  context_parts = previous_parts - resolved_parts + 1
  (previous_script_name.split("/").slice(0, context_parts).join("/")) + new_script_name
end