# File lib/fog/aws/signaturev4.rb, line 110
      def canonical_headers(headers)
        canonical_headers = ''

        for key in headers.keys.sort_by {|k| k.to_s.downcase}
          canonical_headers << "#{key.to_s.downcase}:#{headers[key].to_s.strip}\n"
        end
        canonical_headers
      end