def add_authorization! signer
self.access_key_id = signer.access_key_id
add_param('AWSAccessKeyId', access_key_id)
if signer.respond_to?(:session_token) and token = signer.session_token
add_param("SecurityToken", token)
end
add_param('SignatureVersion', '2')
add_param('SignatureMethod', 'HmacSHA256')
add_param('Signature', signer.sign(string_to_sign))
end