def check_parameters
      errors = ""
      @url.scan(/:(\w*)/).flatten.map(&:to_sym).each do |arg|
        errors += "<li><p>parameter '#{arg.to_s}' is required</p></li>" unless @attributes.has_key?(arg)
      end
      errors.empty? ? "" : "<ul>#{errors}</ul>"
    end
def check_parameters
      errors = ""
      @url.scan(/:(\w*)/).flatten.map(&:to_sym).each do |arg|
        errors += "<li><p>parameter '#{arg.to_s}' is required</p></li>" unless @attributes.has_key?(arg)
      end
      errors.empty? ? "" : "<ul>#{errors}</ul>"
    end
Private: checks the attributes to see if there is any required
          param missing.

 Returns: a list with the error descriptions.