# File lib/httpauth/digest.rb, line 441
      def to_header
        @h[:nonce] ||= Utils.create_nonce @h[:salt]
        @h[:opaque] ||= Utils.create_opaque
        @h[:algorithm] ||= HTTPAuth::PREFERRED_ALGORITHM
        @h[:qop] ||= [HTTPAuth::PREFERRED_QOP]
        Utils.encode_directives Utils.filter_h_on(@h,
          [:realm, :domain, :nonce, :opaque, :stale, :algorithm, :qop]), :challenge
      end