class Rack::Utils::HeaderHash

def self.[](headers)
  warn "Rack::Utils::HeaderHash is deprecated and will be removed in Rack 3.1, switch to Rack::Headers", uplevel: 1
  if headers.is_a?(Headers) && !headers.frozen?
    return headers
  end
  new_headers = Headers.new
  headers.each{|k,v| new_headers[k] = v}
  new_headers
end