createsend step4.2.5a.rb

    # Exchange a provided OAuth code for an OAuth access token, 'expires in'
    # value, and refresh token.
    def self.exchange_token(client_id, client_secret, redirect_uri, code)
      response = request_token(client_id, client_secret, redirect_uri, code)
      fail_on_erroneous_response(response,
                                 "Error exchanging code for access token")
      TokenResponse.from_hash(response)
    end