APPTOKEN is mandatory in python-glpi-api, while optional by GLPI

[AldarisPale]

First of all, thank you for your work on this!

I noticed an aspect. apirest.php states (random internet accessible instance is for example at https://suporte.inss.gov.br/glpi/apirest.php/ ) this:
"App-Token: authorization string provided by the GLPI API configuration. Optional." in the "Init Session" section.

Currently python-glpi-api requires apptoken:

1. python-glpi-api/glpi_api.py

   Line 152 in 47e35a1

   'App-Token': apptoken

2. python-glpi-api/glpi_api.py

   Line 176 in 47e35a1

   'App-Token': apptoken

Leaving aside documentation aspect, the following patches should fix these.

Currently from L148

  # Set required headers.
        headers = {
            'Content-Type': 'application/json',
            'Session-Token': session_token,
            'App-Token': apptoken
        }

To:

  # Set required headers.
        headers = {
            'Content-Type': 'application/json',
            'Session-Token': session_token,
        }
        if apptoken:
            headers['App-Token'] = apptoken

Currently from L174

        init_headers = {
            'Content-Type': 'application/json',
            'App-Token': apptoken
        }

To:

        init_headers = {
            'Content-Type': 'application/json',
        }
        if apptoken:
            init_headers['App-Token'] = apptoken