def refresh!
      raise CredentialsMissingError.new("No credentials to refresh") unless access_token
      raise CredentialsMissingError.new("No refresh_token provided") unless access_token.refresh_token

      do_request do
        @access_token = access_token.refresh!
        Credentials.new(@access_token)
      end
    end
def refresh!
      raise CredentialsMissingError.new("No credentials to refresh") unless access_token
      raise CredentialsMissingError.new("No refresh_token provided") unless access_token.refresh_token

      do_request do
        @access_token = access_token.refresh!
        Credentials.new(@access_token)
      end
    end
Internal: Refreshes the access token

 Returns Hash of token elements to allow client to update in local store
 for user

 Raises Cronofy::CredentialsMissingError if no credentials available.