class Rack::Proxy

def build_header_hash(pairs)
  if Rack.const_defined?(:Headers)
    # Rack::Headers is only available from Rack 3 onward
    Headers.new.tap { |headers| pairs.each { |k, v| headers[k] = v } }
  else
    # Rack::Utils::HeaderHash is deprecated from Rack 3 onward and is to be removed in 3.1
    Utils::HeaderHash.new(pairs)
  end
end