# File lib/aws/core/signature/version_2.rb, line 19
        def add_authorization! credentials
          add_param('AWSAccessKeyId', credentials.access_key_id)
          if token = credentials.session_token
            add_param("SecurityToken", token)
          end
          add_param('SignatureVersion', '2')
          add_param('SignatureMethod', 'HmacSHA256')
          add_param('Signature', signature(credentials))
          self.body = url_encoded_params
        end